X-axis accuracy in two axis machine tools

ABSTRACT

A machine tool, comprising: a worktable assembly slidably carried by the machine frame for reciprocal movement along a work axis and which is configured to carry a workpiece. A worktool assembly is slidably carried by said machine frame for reciprocal movement along a worktool axis at a predetermined angle with respect to said work axis and so as to intersect same. A reference straight edge is carried by said worktable assembly and has a reference surface. Reference sensing means is carried by said machine frame for coaction with said reference straight edge reference surface, to measure reference spacing between said reference surface and said reference sensing means and to provide a reference output indictive of said reference spacing. Control means is provided to facilitate movement of said worktool towards and away from said work axis, which is responsive to said reference output to further facilitate worktool movement towards said work axis. The reference sensing means includes a fixed electrode that forms with said reference surface a capacitance gauge. The control means includes a look-up table storing information relative to initial dispositions of said reference surface and said reference sensing means, to provide an initial disposition output indicative thereof to further modify and facilitate worktool movement towards said work axis. The look-up table initial disposition output is indicative of the flatness of said reference surface.

FIELD OF INVENTION

This invention concerns machine tools particularly apparatus forgrinding piece parts and other components to a very high accuracy,particularly but not exclusively cylindrical components such as spindlesand shafts.

BACKGROUND TO THE INVENTION

Due to forces exerted on a workpiece, the worktable and workpiecesupporting structures of such machines, distortions and misalignmentsoccur. These result in inaccuracies in the dimensions and shape of thefinal machined article. Where the dimensional errors are of the sameorder as the accuracy to which the article is to be machined it isimpossible reliably to machine the article. Distortions misalignmentsand other error producing effects which can arise and tend to becumulative must therefore be reduced to a level which is such that themagnitude of the cummulative error in the final article is much lessthan the error which can be permitted therein.

One of the primary controlling parameters in a machining operation isthe accuracy with which the workpiece position relative to a machiningtool is known and it is an object of the present invention to provide asystem by which the position o the worktable and the workpiece thereoncan be accurately determined relative to a machining tool so that if thelatter can be moved in a precise manner to engage the workpiece,material can be removed therefrom to a known depth. When applied to amachine tool incorporating features such as are described in ourcopending applications (our references C421.01/L and C422.01/L toC427.01/L) a highly accurate machine tool can be obtained.

The invention is of particular relevance to a cylindrical grindingmachine.

PRIOR ART

From Japanese patent publication No. JP-A-2160457 and No. JP-A-54102671are known machine tools having straight edges and sensor meanscooperating therewith, aimed at compensating for errors in accuracy inthe path of movement of a machine drive.

SUMMARY OF THE INVENTION.

According to one aspect of the invention there is provided a machinetool comprising:

(a) a machine frame;

(b) a worktable assembly slidably carried by said machine frame forreciprocal movement along a work axis and which is configures to carry aworkpiece whereby the worekpiece is movable along the work axis;

(c) a worktool assembly slidably carried by said machine frame forreciprocal movement along a worktool axis at a predetermine angle withrespect to said work axis and so as to intersect same, with the worktoolmovable along the worktool axis;

(d) a reference straight edge carried by said worktable assembly andhaving a reference surface;

(e) reference sensing means carried by said machine frame for coactionwith said reference straight edge reference surface, to measurereference spacing between said reference surface and said referencesensing means and to provide a reference output indictive of saidreference spacing; and

(f) control means to facilitate movement of said worktool toward andaway from said work axis which is responsive to said reference output tofurther facilitate worktool movement towards said work axis, where thereference spacing is measured at height above the worktable which iscommensurate with the height of the work axis.

Advantageously a point in the reference surface at which the referencespacing is measured, the work axis and the worktool axis all lie in thesame plane.

Typically said worktool assembly includes a worktool carriage movablydisposed on a slideway carried by said machine frame for movement alongsaid worktool axis and worktool carriage position sensing meansproviding an output signal indicative of the position of said worktoolcarriage to said control means to further facilitate movement of saidworktool towards said work axis.

The worktool carriage position sensing means is preferably located atthe same height as the said point in the reference surface at which thereference spacing measurement occurs.

Preferably the worktool carriage position sensing means and thereference spacing measuring point are aligned wish the worktool axis.

The invention is of particular application to a machine in which theworktool is a grinding wheel rotatable about a wheel axis and thereference spacing measurement point, workpiece axis worktool axis andthe wheel axis all lie in the same plane.

The machine is preferably mounted so that the said one plane horizontal.

Conveniently said control means includes a look-up table storinginformation relative to initial dispositions of said referee surface andsaid reference sensing means and to provide initial disposition outputindicative thereof to modify s reference output to further facilitateworktool movement towards said work axis.

Preferably said look-up table initial disposition output indicative ofthe flatness of said reference surface.

Conveniently said reference sensing means includes a fixed electrodethat forms with said reference surface a capacitance gauge.

Preferably a second capacitance gauge is disposed close to the firstcapacitance gauge and a capacitance bridge is provided including thefirst and second capacitance gauges, to provide a capacitance variationsignal.

Preferably said worktable assembly includes a work carriage movablydisposed on slideways carried by said machine frame for movement alongsaid work axis, and said worktool assembly carries a worktool, a workingface of which is positionable thereby with respect to said work axis;said reference spacing being indicative of variations in spacing betweensaid working face of said worktool and said work axis due to shifting ofsaid wore carriage with respect to said work face of said work tool.

Conveniently said worktool carriage position sensing mean includes agrating disposed on said worktool carriage in worktool imaginaryreference line that intersects work when carried by said worktableassembly.

A rigid cover is to advantage mounted on the worktable to protect thereference sensing means, the cover being spaced from the latter.

A second rigid cover is preferably mounted on the worktool carriage,spaced from the position sensing means to protect the latter.

Prefarably there is provided a cover assembly for said slideways mountedto said machine frame movable relative to and responsive to movement ofsaid work carriage but not connected to said work carriage.

The cover assembly preferably includes a first cover port mounted tosaid machine frame to one side of said work carriage and a second coverportion mounted to said machine frame to the other side of said workcarriage.

The invention is of particular application to a machine in which thetool is a grinding wheel.

According to another aspect of the present invention a machine toolcomprises a bed, a worktable movable relative to the bed along a linearpath, a tool movable to engage a workpiece on the worktable, tool drivemeans for advancing and retracting the tool along a tool path generallyorthogonal to the said linear path, worktable drive means for effectingthe movement of the worktable relative to the bed along the linear pathand therefore relative to the tool, a reference straight edge carried bythe worktable, reference sensing means fixed on the machine bed andstationary relative to the straight edge and spaced therefrom by anominal distance generally at the level of the path of movement of theworkpiece, and cooperating therewith to produce an electrical signal andcircuit means associated therewith to generate from said electricalsignal an error signal indicative of any departure of the distancebetween the straight edge and the sensing means from the nominal spacingtherebetween, memory means for storing a calibration of the flatness ofthe straight edge for points along its length, electrical signalgenerating mean for generating a target position signal for the toolalong the X-axis (the X-axis target signal), tool position sensing meansfor generating an electrical signal indicative of the actual position ofthe tool along the X-axis (the feedback signal), further electricalcircuit means responsive to the electrical signal from the referencesensing means and to the calibration signal from the memory meansthereby to influence either the X-axis target signal, or the feedbacksignal, so as to enable the tool drive means to position the tool alongthe Z-axis in a manner which compensates for any variation in thespacing between the reference straight edge and the reference sensingmeans caused by factors such as worktable yaw and roll.

Preferably the straight edge is formed from an elongate block of ceramicmaterial.

Preferably one face of the block is coated with a conductive material toform an electrode, and the sensing means includes a conductive platespaced from the conductive face of the block and forming therewith acapacitance the value of which will vary with movement of the worktablerelative to the fixed conductive plate if the trajectory of theworktable is not a straight line.

The conductive film is preferably hard chrome.

The machine as aforesaid advantageously further comprises a referencecapacitance of substantially the same nominal capacitance as that of thesensing capacitance formed between the conductive electrode andconductive surface of the reference straight edge, the referencecapacitance being positioned close to the sensing capacitance so thatenvironmental variations such as changes of temperature and humiditywhich can alter capacitance can be balanced out by comparing the twocapacitances.

Conveniently the two capacitances form part of a bridge circuit so thatit is only signals indicative of changes of capacitance of the sensingcapacitance relative to the reference capacitance which are transmittedas output signals indicative of variation of capacitance due to nonstraight line motion of the worktable.

Preferably one of the electrodes of the reference capacitance comprisesa hard chrome metallising of a surface of a block of ceramic materialsimilar to that forming the reference strait edge.

As applied to a grinding machine the invention provides a machine inwhich the machining tool is a grinding wheel and the workpiece ismounted between headstock means and tailstock means mounted on theworktable and in which at least the headstock is rotated by drive meansto rotate the workpiece. In such a machine the grinding wheel is mountedon a wheelhead slidable along an axis (the wheelhead axis) perpendicularto the axis of linear movement of the worktable and positioned generallyopposite the position of the sensor which cooperates with the straightedge on the worktable, and wherein a wheelhead servo control system issupplied with a target signal defining the desired position of thewheel, along the wheelhead axis, signals obtained from the referencesensing associated with the reference-straight edge, for combining witha feedback signal indicating the actual position of the wheelhead,whereby the wheelhead is positionable along the wheelhead axis relativeto the worktable and therefor the workpiece, taking into account anyalteration in the spacing between the reference sensing means and thereference straight edge caused by Z-axis imperfections such as table yawor roll.

The machine as aforesaid preferably further comprises look-up tablememory means containing calibration data relative to length of thereference straight edge, relating to linear variation of the saidspacing caused by non-parallel mounting of the straight edge on theworktable and/or worktable and slideway, and/or X-axis imperfections(which may be non-linear) dependent on the position of the wheelhead onthe X-axis.

The invention also provides a method of controlling the position of amachining tool relative to a workpiece mounted on a worktable itselfslidable in a straight line along a worktable axis (the Z axis) relativeto a machine bed, in a direction generally perpendicular to thedirection of movement of the machining tool, the worktable carrying areference straight edge which cooperates with sensing means fixed to themachine bed and located generally opposite the point of application ofthe tool generally at the level of the workpiece axis, for determiningany non straight line movement of the worktable, comprising the stepsof, storing a calibration signal in the form of a look up table for aplurality of positions of the worktable along the Z-axis; generating anerror signal indicative of any non straight line movement of theworktable; adjusting the error signal using the stored calibrationsignals; and adjusting a signal defining the target position for thetool along the X-axis (or the feedback signal indicating the actualposition of the tool), by the adjusted error signal, thereby to causetool positioning means to position the tool relative to the worktableand any workpiece positioned thereon taking into account errorsaffecting the accuracy of the tool position relative to the worktableaxis such as yaw or roll or non parallelism of the straight edge to theZ-axis.

The method is applicable to a machine in which the tool is a grindingwheel mounted on a wheelhead which is slidable in a direction generallyperpendicular to the direction of movement of the worktable.

The reference straight edge is preferably electrically conductive andforms with a fixed electrode a capacitance which varies if the worktabledoes not follow a straight line path and wherein circuit means isprovided for generating an electrical signal indicative of anycapacitance variation thereby to generate an error signal forinfluencing the value of a wheelhead target position signal or awheelhead position feedback signal to enable the wheelhead positionmeans, to adjust the position thereof, relative to the worktable, totake account of the lateral movement of the worktable sensed by thereference straight edge edge sensing means.

The invention also lies in apparatus for detecting unwanted lateralmovement of a worktable movable relative to a machine bed along aworktable axis comprising a straight edge carried by the worktable and aconductive probe fixed to the machine bed and cooperating with thestraight edge, generally at the level of the path of movement of theworkpiece, to form a capacitance which varies with any lateral movementof the worktable but maintains a predictable value with movement of theworktable along the said available axis, and electric circuit meanscoupled to the said capacitance to generate an electric signalindicative of unpredicted variations in the said capacitance to providean electric signal indicative of any lateral movement of the worktablerelative to the worktable axis.

Such apparatus may further comprise means responsive to the saidelectrical signal derived from the capacitance variation to adjust theposition of a tool adapted to engage a workpiece carried on theworktable so as to compensate for unwanted lateral movement of theworktable.

A reference capacitance is preferably situated in close proximity to thecapacitance formed between the straight edge and the sensing means toenable variations in capacitance of the latter due to environmentalchanges to be balanced out.

The invention also lies in a method of correcting errors due to unwantedlateral movement (such as yaw and roll) of a worktable motion in amachine tool in which the worktable for carrying the workpiece ismovable along a straight line path (the worktable axis) relative to amachine bed, the worktable carries a reference straight edge, generallyat the level of the path of movement of the workpiece, and the machinebed a sensor adapted to cooperate with the reference straight edge andform a transducer therewith which will vary an electrical signal in theevent of lateral movement of the worktable along its path, the methodcomprising the steps of monitoring the said electrical signal andgenerating an error signal dependent upon any variation thereon, thesaid error signal indicating in magnitude and sign the lateral movementof the worktable from the straight line path intended for it to follow,and using the error signal to control the final position of a toolmovable relative to the worktable and therefore any workpiece locatedthereon.

The transducer may comprise a capacitance formed between conductivesurfaces on the reference straight edge and a final electrode andfurther comprising the step of providing a second capacitance theelectrodes of which are located in close proximity to the firstcapacitance, to provide a reference capacitance for comparison with thefirst said capacitance and monitoring both said capacitances to enableenvironmentally induced changes in the said first capacitance to bebalanced out by the same induced changes in the reference capacitance.

The invention also lies in workpieces when produced by the machinetools, and the product of methods, described herein.

The invention will now be described with reference to the accompanydrawings:

FIG. 1 is a front elevation of a grinding machine embodying theinvention;

FIG. 2 is an end view of the machine of FIG. 1;

FIG. 3 is a plan view of the machine of FIG. 1 with some parts removedfor clarity;

FIG. 4 is a plan view of the machine with its surrounding controls andpower supplies;

FIG. 5 is a flow diagram of the cooling system of the machine;

FIG. 6 is a schematic diagram of the essential operating parts of themachine;

FIG. 7 is a block schematic diagram of the electrical monitoring andcontrol system of the machine;

FIG. 8 is a schematic diagram of the closed hydraulic circuits of themachine;

FIG. 9 is a schematic flow diagram of the controls and supply of fluidto the linear bearings and leadscrew bearings in the machine;

FIG. 10 is a similar diagram for the rotating bearings of the machine;

FIG. 11 is a block schematic diagram of the workpiece coolant system ofthe machine;

FIGS. 12 to 16 illustrate different grinding techniques

FIG. 17 is a perspective view of the grinding machine of FIGS. 1 to 3with all the covers in place and with the controller cabinet 620 inplace;

FIG. 18 is a similar view to that of FIG. 17, with some of the coversremoved;

FIG. 19 is a partially diagrammatic cross-section through the worktableregion of the machine shown in FIG. 18, in the direction of arrow A;

FIG. 20 is a similar cross-section through the machine of FIG. 18 in thedirection of arrow B;

FIG. 21 is a true cross-section through the worktable showing how thelatter is carried by hydrostatic bearings on two ceramic rails forming aslideway;

FIG. 22 is a cross-section (on line AA, see FIG. 23B) through theassembly which is bolted to the top right hand corner of the casting ofFIG. 21, and which contains the ceramic straight edge for accuratelydelivering the X-axis position of the worktable relative to the machineframe, and the grating for determining the Z axis position of theworktable relative to the machine frame;

FIGS. 23A/B is an elevation of one end of the assembly shown in sectionin FIG. 22;

FIGS. 24A/B is a plan view of the assembly shown in FIGS. 23A/B;

FIG. 25 is a cross-section on the line BB of FIG. 24B;

FIG. 26 is an elevation partly in section and partly cut away, showingend and intermediate mounting detail of a rod which is engaged by thepinch rollers of a friction drive mounted at the left hand end of theworktable for driving the worktable from side to side;

FIG. 27 is a plan view of the rod arrangement of FIG. 26;

FIG. 28 is a typical cross-section through the cover which is fittedover the assembly shown in FIGS. 22-24, and is secured to the worktable;

FIG. 29 is an elevation of the left hand end of the machine with theworktable located nearer the left than the right hand end of itstraverse, and showing a set of telescoping covers protecting the lefthand end of the worktable and the friction drive engaging the rod ofFIGS. 26 and 27;

FIG. 30 is a plan view in the direction of arrow A in FIG. 29;

FIG. 31 is an elevation sectioned on the line YY of FIG. 30.

FIG. 32 is an elevation partly in section of the right hand end of themachine, and shows the right hand end of the worktable and right handbulkhead to which the other set of telescoping covers which protect theright hand end of the worktable slideway are attached, and also thedrive to the bulkheads and covers;

FIG. 33 is a cross-section through the worktable (on the line AA in FIG.34), which extends between the right hand end of FIG. 29 and the lefthand end of FIG. 32;

FIG. 34 is an elevation through the worktable with the central sectionremoved, showing detail of the inboard termination of the left hand endof the bulkhead drive;

FIG. 35 is a section on line EE of FIG. 32, giving details of thetelescoping cover at the right hand end of the machine;

FIG. 36 is an elevation view, (partly cut away to reduce the overalllength), of the potentiometer device for tracking the position of thebulkhead attached to the right hand covers, as seen in the direction ofarrow B in FIG. 35;

FIG. 37 is a section showing how the right hand end of a rigid tubewhich joins the two bulkheads together is secured to the right handbulkhead;

FIG. 38 is an elevation in cross-section of the right hand end of theworkhead assembly;

FIG. 39 is an elevation, in a plane at right angles to the cross-sectionplane of FIG. 38, of the left hand end of the workhead assembly;

FIG. 40 is a cross-section through the housing at the right hand end ofFIG. 39 on the line RR; in FIG. 41;

FIG. 41 is an external elevation of the right hand end of the wheelheadassembly not visible in FIG. 39;

FIGS. 42 A an B are a section through the tailstock assembly;

FIG. 43A is an underside view of the tailstock and 43B is a partialrighthand end view;

FIG. 44 is a cross-section through a hydrostatic bearing in FIG. 42;

FIG. 45 is a general assembly, partly cross-sectioned, of a wheel headassembly for use in the machine of FIGS. 1 to 4;

FIG. 46 is a plan view of the wheelhead assembly of FIG. 45;

FIG. 47 is a section on the line AA of FIG. 46;

FIG. 48 is a cross-section through a side elevation of part of ahydrostatic head screw drive for advancing and retracting the grindingwheelhead assembly mounted on the platform of the drive shown in FIGS.45 to 47;

FIG. 49 is a similar view of the remainder of the head screw drive ofFIG. 48;

FIG. 50 is an end elevation, partly cut away, of the drive of FIGS. 48,49;

FIG. 51 is a cross-section on the line XX of FIG. 48;

FIG. 52 is a cross-section through part of a wheelhead drive unit forthe grinding wheel of the machine of FIGS. 1 to 4;

FIG. 53 is a continuation of the section of FIG. 52;

FIG. 54 is a cross-section on the line AA of FIG. 53;

FIG. 55 is a side elevation of the x-axis measuring device for thewheelhead assembly of the machine of FIGS. 1 to 4;

FIG. 56 is an end elevation of the device shown in FIG. 55;

FIG. 57 is a plan view of the device shown in FIG. 55;

FIG. 58 is a cross-section through the central vertical spindleincorporating a lifting/indexing facility for the wheelhead assembly ofFIGS. 45 to 47;

FIG. 59 is a plan view of the wheelhead swivel drive unit, as shown inthe wheelhead assembly of FIG. 46;

FIG. 60 is an elevation of the drive unit of FIG. 59;

FIG. 61 is a cross-section through one of two diameter measuring gauges(such as Movomatic gauges), mounted on the worktable slideway to assistin size control and in obtaining parallel grinding;

FIG. 62 is a side elevation of a wheel guard assembly for fitting to themachine of FIGS. 1 to 4;

FIG. 63 is a front elevation of the assembly of FIG. 62;

FIG. 64 is a schematic arrangement showing how a coolant nozzle can beaccommodated onto the assembly of FIGS. 62-63;

FIG. 65 is a front elevation similar to that of FIG. 63 with the nozzleshown in position;

FIG. 66 is a side elevation of a support assembly and a wheel formingunit, mounted on the headstock housing;

FIG. 67 is an end elevation of the assembly of FIG. 66;

FIG. 68 is a side view of an electrolytic wheel dressing device adaptedto be mounted on the top face of the wheelguard of FIG. 62;

FIG. 69 is a side elevation of a shoulder touch probe for us with themachine of FIGS. 1 to 4;

FIG. 70 is a block schematic diagram showing how the grinding wheelforces acting on the workpiece can be resisted by an active worksteadycontrolled by signals form the headstock and tailstock, and

FIG. 71 is a schematic diagram showing how an error signal from thecapacitance gauge can be used to influence the X-axis wheelheadposition; and

FIG. 72 is a similar schematic diagram showing an alternative way ofinfluencing the X-axis wheelhead position not only from the capacitancegauge but from other error signal inputs.

DETAILED DESCRIPTION OF THE DRAWINGS

I General Overview

The complex machine shown in the drawings will be described in detailwith reference to the different sections of the machine which make upthe whole. However by way of introduction a general overview of themachine will be gained by referring to FIGS. 1 to 4.

The machine shown in the drawings comprises a cylindrical grindingmachine capable of grinding to a very high accuracy typically of theorder of a few nanometers. In order to achieve such accuracies, controlof the workpiece, the wheelhead and the grinding wheel must be veryprecise and vibration which can arise during operation of the machineand which can be transmitted to the machine by outside influences mustbe reduced to a very low level. Failure to isolate the workpiece andgrinding wheel from vibration whether internally or externallyoriginating, will prevent the machine from performing to the high levelof accuracy desired.

FIG. 1 is a side elevation of the machine viewed from the side on whichan operator would stand. The machine base is of such size that thewheelhead and workpiece region of the machine would be out of reach to ahuman operator if the latter were to stand on the same floor as the baserests. To this end the machine base is shown sitting on a foundationfloor 10 and the floor on which an operator stands is denoted by 12. Thelatter is apertured to permit the base structure to extend freelythrough the floor 12.

An intermediate support frame 14 of generally triangular outline whenviewed in plan, is carried by vibration isolators, two of which arevisible in FIG. 1 at 16 and 18. Vibration isolation brakepointfrequencies are selected as 2.5 Hz for vertical components of vibrationand 5.0 Hz for horizontal components of vibration.

The foundation floor 10 is typically constructed from concrete.

The machine frame generally designated 20 is typically formed fromPolymer concrete typically ACO Polymer concrete, and supports at theleft hand end a workhead assembly 22, at the right hand end a tailstockassembly 24 for supporting therebetween a workpiece generally designated26 for grinding by a grinding wheel designated 28 carried in a wheelheadassembly generally designated 30. The frame 20 (commonly called themachine bed) is mounted in the frame 14 three feet assemblies.

II Services for Workhead and Tailstock

Hydraulic, pneumatic and electrical power, cooling fluid and the likeare conveyed via umbilical tubes 32 between the bed and the tailstockand 34 (between the bed and the guard assembly).

Housing 36 contains a forming wheel advance/retract mechanism which willbe described in more detail later.

Housing 37 contains an electrolytic wheel dressing device by which thegrinding wheel 28 can be referred periodically, as required.

Also visible in FIG. 1 are telescoping shrouds 38 and 40 the purpose ofwhich is to protect the slideways on the worktable, a for the headstockand tailstock, see section XI below.

In FIG. 2 a third umbilical 42 conveys services from the machine housingto the wheelhead assembly 30 which is movable towards and away from theworkpiece 26 and slideway 44. Also visible is the section through theworktable 46 on which the workhead and tailstock assemblies slide. InFIG. 2 the tailstock assembly is missing thereby revealing the workpiece26 and workhead assembly 22 and wheel dressing housing 36.

The telescoping shrouds 38 can be seen below the worktable 46 as can thesegmented umbilical 34 providing services to the headstock assembly.

A third of the four isolating and levelling feet on which the frame 14is mounted can be seen at 48, the fourth one being hidden from view.

FIG. 3 is a plan view of the overall machine which shows in hiddendetail four isolating and levelling feet 16, 18, 48 and 50 and also byway of hidden detail the three mounting points between the machine frame20 and the intermediate frame 14 shown at 52, 54 and 56.

III Main Services for the Machine

These are provided via ducting 58 and feedback and control cabling andpiping communicates via the same ducting and a control console 60positioned to the left of the main operator workstation designated by A.A second operator position is shown at B.

A further umbilical 62 serves to convey services directly to thewheelhead assembly 30 from a rearwardly mounted section of the wheelheadassembly. Grinding disc is shown in dotted outline and is designated 28as before and the drive motor for the disc is shown at 64.

FIG. 4 shows the machine in relation to the power supply cabinets,control system cabinets, machine coolant pumps and hydrostatic tanks.The same reference numerals are used as have been employed in earlierfigures to denote similar parts.

Electrical power supply and control system cabinets are shown at 66 and68 and additional ducting at 70 conveys cabling between the controller60 and the cabinets 66 and 68.

Overhead extraction of fumes and removal of air for cooling is effectedby means of overhead ducting 72 and 74. Extractor fans or the like areprovided (not shown).

In addition to the electrical cabinet 68, electrostatic cleaning for thefluid from the linear bearings is provided in cabinet 76 together withthe closed hydraulic system transformer. Electrostatic cleaning of fluidfrom the rotating bearings is contained in cabinet 78 together withdouble refrigeration unit and hydrostatic control of the fluid to andfrom the linear bearings and rotating bearings is provided in cabinet80.

Filters for the various hydraulic and where appropriate pneumaticcircuits are included in cabinet 82.

Coolant for supply to the workpiece and a double refrigeration unit areincluded in cabinet 84 and coolant pumps are contained within cabinet86.

Tanks 88 and 90 contain hydrostatic oil, 92 contains machine coolant and94 workpiece coolant. Pipework between the tanks and the respectivecabinets 76, 78, 80, 82, 84 and 86 is provided together with pipeworkfrom the cabinets to the machine and from the machine to the tanks.

The cabinets 68 to 86 are conveniently located behind a wall designated96 and where appropriate ducts are provided through the wall for theinterconnection of services.

IV Cooling

Cooling of the various parts of the machine which generate heat in useis effected by pumping fluid (typically chilled water) through coolinglabyrinths in the spindle housing, headstock, wheelfeed and tailstock.The rate at which heat can be removed is controlled by maintaining aconstant reduced inlet temperature and individually varying the flowrate to each controlled area on the machine using closed looptemperature controllers. An inlet temperature of 18.5° C. will allow upto 3.5 Kw of heat to be removed using a flow rate of 32 liters perminute of water from each part of the machine if the exit temperature isto be 20° C.

The cooling of the machine will be described in greater detail withreference to FIG. 5. This shows the four active sites, namely theworkhead 22, the wheelhead 30, the wheelfeed 41 and the tailstockassembly 24.

Fluid is supplied to each of the four active sites via flow controlvalves 98 to 104 and pressure is maintained by line pumps 106 to 112.Each of the flow control valves is independently controlled by a signalF1, F2 etc from each of four temperature controllers 114 to 120, andtemperature signals for the controllers are developed by platinumresistance probes 122, 114, 126 and 128 respectively associated with theworkhead, wheelhead, tailstock and wheelfeed respectively. Each of thetemperature probes senses the temperature of the fluid leaving each ofthe respective devices. The flow control valves 98 to 104 serve tocontrol the rate of flow of coolant fluid along feeds 130, 132, 134 and136 to the workhead, wheelhead, tailstock and wheelfeed respectively andunwanted coolant fluid is returned via dump lines 99, 101, 103 and 105to a common return line 138 feeding unwanted coolant fluid to thecollection tank 140.

After passing through each of the workhead, wheelhead, tailstock andwheelfeed respectively, the coolant fluid is returned via a commonreturn path 142 to the collection tank 140.

A one-way valve 144 prevents suck-back into the tank and a master pump146 serves to deliver fluid from the tank 140 to the refrigeration unit148. The latter delivers cooled fluid to the line 150 feeding the linepumps 106 to 112 and the temperature of the fluid in line 150 is sensedby a platinum resistance temperature probe 152. The signal developed bythe latter controls a temperature controlling device 154 which in turncontrols the operation of the refrigeration unit to maintain thetemperature in line 150 constant.

Typically the latter is controlled to 18.5° C. and each of the pumps iscapable of delivering 32 liters per minute to each of the workhead,wheelhead, tailstock and wheelfeed respectively. The actual quantity offluid supplied to each is controlled by the flow control valves aspreviously described so as to maintain the outlet temperature of thecoolant fluid from each of the workhead, wheelhead, tailstock andwheelfeed respectively constant. Typically the exit temperature iscontrolled to 20° C. so that only 1½° C. rise occures as the coolantpasses through each of the components.

It will be seen that within the capabilities of the pumps 146 and 106 to112, each of the workhead, wheelhead, tailstock and wheelfeed can becontrolled in temperature irrespective of the head developed duringoperation within each of said units.

Since the system is essentially closed loop, and since the volume of thereturn paths 99 to 105 and 138 is relatively small as compared with thecoolant system volume associated with the feeds and returns and activesites, increased demand for cooling will result in less fluid beingreturned via path 138 and more fluid being tied up in the coolingpassages associated with the active sites. The level in the tank willdrop and this can be used to trigger an alarm and machine shut-down inthe event that the level drops below a given threshold.

Monitoring the individual temperatures can also be used to instigatemachine shut-down in the event of temperature overrun.

The level sensing loop additionally safeguards coolant fluid loss due toleakage or otherwise.

V Block Schematic of Overall Machine

FIG. 6 is a schematic block diagram which indicates the essential partsof the grinding machine some of which have already been referred to.

Essentially the workpiece (not shown in FIG. 6) is located between theworkhead and tailstock and both are driven in the same sense and at thesame speed and in phase so that no torque is developed across theworkpiece due to frictional drag at a stationary tailstock. To this endboth workhead and tailstock include a drive motor 156 and 158 and aresolver 160 and 162 respectively. Speed and radial position of theworkhead drive are developed by a tacho 164 and an encoder 166. Thetailstock motor is slaved to the workhead motor.

The workhead and tailstock are maintained in fixed relationship on aworktable 168 which itself is slidable linearly in the direction of theZ-axis shown at 170. Z-axis drive is achieved by means of a frictiondrive unit indicated diagrammatically at 172 as cooperating with asmooth rail 174. The drive is rotated by means of a motor 176 the speedof which is indicated by means of a tacho 178.

Linear position of the worktable is obtained by signals from an opticalreading head 180 operating in conjunction with a linear scale in theform of a grating 182. Positional information is available via signalpath 184. Although shown distant from the point at which the wheelengages the workpiece, the optical reading head cooperates with thegrating is preferably arranged so as to be as near as possible in linewith the wheel. Ideally the grating scale 182 should be read at aposition in line with the wheel. However, the X-axis correctioncapacitance measuring device (to be described) will suffer from muchmore serious errors if it is not in line with the wheel and where theworktable is relatively long and the scale 182 is therefore alsorelatively long it has been found that provided the Z-axis reading head180 is in line with the wheel it can be located below the planecontaining the workpiece and wheel axes without machine errors arising.

The wheelhead and grinding wheel mounted thereon is driven by a DCbrushless motor 64 commutated by a brushless resolver integral withinthe spindle assembly at 186. A gap sensor 190 and associated gap controldevice 192 are provided.

The motor and wheelhead assembly 64, 28 is mounted on a turntable topermit rotation of the wheel axis about a vertical axis orthogonal tothe motor axis. Part of the turntable is shown at 194 and a rectilinearpotentiometer 196 provides rotational positional information along 198to the system control (to be described).

Wheel balancing is effected through a wheel balance control system 200.

VI Wheelhead Indexing

The turntable is rotatable relative to a support and both havecooperating rings of gear teeth which engaged to hold the turntable inany selected position, but can be disengaged by lifting the turntablerelative to its support, to allow indexing to occur.

A rectilinear potentiometer 202 provides an output signal to the systemcontrol (to be described) indicating when the turntable has been liftedby a lifting mechanism (not shown) clear of the indexing teeth (to bedescribed).

Indexing of the turntable 194 is achieved by a drive motor 204 androtating resolver 206 after the turntable has been lifted. Thearrangement of the teeth associated with the turntable and its supportpermit N equally circularly spaced positions to be accurately defined.The lifting and indexing mechanisms will be described in more detailwith reference to FIG. 6.

Normal cylindrical grinding will be achieved using the wheel with itsaxis parallel to the Z-axis. However where a different angle of attackis required, indexing the turntable to the desired angular position willcause the grinding wheel to present to the workpiece at the correctangle.

VII Wheelfeed

The turntable itself is mounted on a table or wheelslide 208 (item 44 inFIG. 2) which itself slides on the slideway 43 shown in FIG. 2.Wheelfeed, that is movement of the wheelhead towards and away from theworkpiece, is achieved by means of a lead screw type drive 210 driven bya motor 212 with a commutating resolver 214 and associated tacho 216.The output from the latter provides the input signal to a velocitycontrolled servo-system. Although final positioning is achieved using alinear grating (see Section VIII) initially, until the wheelheadapproaches the desired position, it is the wheelfeed is velocity whichis controlled and the motor 210 is rotated at a speed for a given lengthof time which can be computed to move the wheelhead through a givendistance either towards or away from the workpiece.

VIII X Axis Position Measurement

A linear grating (scale) 218 is carried by the wheelslide 208 and anoptical reading head 220 is fixed to a stationary part of the machine toprovide an electrical signal along line 222 from which the position ofthe wheelslide can be determined. The direction of movement of thewheelslide is commonly referred to as the X axis of the machine asspecified by reference numeral 223. The positional information from theencoder 220 thus corresponds to the position along the X axis. Thelinear scale 218 is mounted at grinding wheel centre height and as nearin line as possible to minimise offset and therefore minimise errors.

A rigid cover fitted to the wheelslide 208 (not shown) to protect thesettings of the optical reading head.

IX X Axis Correction

Since it is an objective of the machine design to permit workpieces tobe ground to an accuracy of a few nanometers, it is important that theprecise position of the workpiece axis is also known to a level ofaccuracy greater than that of the grinding process. The axis can bedefined accurately using precise end bearings in the workhead andtailstock but since the work table 168 has to be capable of slidingalong the Z-axis 170, a working clearance must be provided to enable thesliding to take place and in order to compensate for yaw and rollerrors, a reference straight edge 224 is provided mounted on the worktable 168. The straight edge is preferably formed from ceramic.

The position of the reference straight edge relative to an electrodefixed to the machine forming a capacitance gauge 226, is determined, andan electrical signal is developed by a gauge conditioning unit 228 forsupply to the overall system control along line 230. As with the grating182, the gauge 226 is preferably located close to the point ofwheel/workpiece engagement. The face of the straight edge whichcooperates with the electrode is metallised as by a hard chrome coating.

The wheel, work and capacitance gauge are at the same height so as toprovide accurate compensation, and the capacitance gauge is “in line”with the wheel when the latter is square to he workpiece.

Any movement of the work table 168 (and therefore the reference straightedge) perpendicular to the Z axis, relative to the machine will registeras a change in capacitance seen by the gauge conditioning unit 228, andan appropriate correction signal can be generated to indicate thelateral shift of the worktable 168.

Since the reference straight edge itself may not be perfectly straightand flat and may not be mounted absolutely parallel to the Z axis slide,the invention provides for an initial calibration step in which thestraightness (or lack of straightness) is determined and stored in amemory as a look up table relative to position along the length usingone end as a datum, and a second level of calibration in which theworktable 168 is moved from one end of its travel to the other and anycapacitance variation as measured by the gauge 226 is recorded andstored as a second look up table against Z-axis position. The look-uptable calibration signals and available to correct the capacitance gauge226 reading-for each Z axis position of the reference straight edgeduring subsequent machining operations. In this way the measured valuefrom the capacitance gauge 226 supplied to the overall system controlalong line 230 is corrected for any non-flatness and non parallelism ofthe reference straight edge.

The calibration process described above may of course be replicated fordifferent temperatures within the normal restricted temperature rangeover which the machine is expected to work and further look-up tablesprovided so that not only Z-axis position but also machine temperatureis taken into account in determining the calibration value to be used atany point in time during subsequent machining.

In addition the calibration process may be repeated a number of timesand a mean value for each Z position determined for storage in thelook-up table.

Since the capacitance changes will be very small the sensitivity of thecapacitance gauge can be substantially increased by using a capacitancebridge technique and in this case a second capacitance gauge must beprovided having a fixed capacitance substantially equal to that of thecapacitance of the first gauge 226. The second capacitance gauge isshown at 232 and typically is mounted in close proximity to thecapacitance gauge 226 on the same part of the overall structure so thatenvironmental influences such as humidity and temperature which couldaffect the absolute capacitance of the gauge 226 will also affect thegauge 232 and will be cancelled out. Although not shown a capacitancebridge circuit is created using the two capacitance gauges 226 and 232so that the conditioning unit 228 looks at the difference between thetwo capacitances rather than trying to measure the absolute change ofcapacitance in one or the other.

Although not shown in FIG. 6, a metal cover of rigid and substantialproportions is mounted on the worktable so as to enclose the capacitancegauges. The purpose of the cover is partly to electrically shield thegauges again stray capacity (hand capacity of an operation is sufficientto alter the capacitance value readings but also to prevent an operatorfrom touching and moving the capacitance gauges and upsetting theircalibration.

X Workpiece Diameter Sizing

Sizing of a workpiece during grinding is effected by means of diametermeasuring, gauges 234 and 236. Typically Movomatic gauges are employedand electrical signals therefrom are supplied to a gauge conditioningunit 238 for supply to the overall machine control system via line 240.

XI Slideways and Protective Shrouds

The worktable slideway on which the headstock and tailstock are carried,is preferably formed from ceramic and needs to be protected. Inparticular it is important to keep grinding wheel cooling fluid frommixing with oil used to lubricate the slideway. Covers 38 and 40 areprovided as described with reference to FIG. 1. The covers aremulti-section telescoping arrangements so that movement of the worktablealong the Z-axis can be followed by the cover assembly. However sincethe sliding and telescoping of the various sections making up the covers38 and 40 can introduce vibration and unwanted errors in the Z-axis, thecovers 38 and 40 are mounted independently of the workhead and tailstockassemblies so that although they cover the worktable slideway for thelatter, they are physically separated from the worktable. In order toprovide for movement, the right hand cover assembly 242 is provided witha motor 244 (see FIG. 6). A commutating resolver 246 is associated withthe motor. A rectilinear potentiometer associated with the assembly 242is denoted by reference numeral 248. This provides positionalinformation to the system control. The left hand cover assembly isjoined to the right hand cover assembly so that movement of the latteris followed by the former and as the worktable moves left or right thecovers follow to cover the slideway.

XII Control System

FIG. 7 is a block schematic diagram of the control system including theoperators console of the grinding machine shown in FIGS. 1 to 6.

Some of the parts of the control system generate data which is thenrequired by or used to control other parts of the control system andmachine and to this end a common VME bus 246 is provided as a main datahighway to which all of the intercommunicating separate parts of thecontrol system are connected. The control system integers will be listedbelow.

A first central processing unit 248 provides the main processing powerfor retrieving and processing information.

The second central processing unit 250 provides control signals via aserial link 252 to the wheel balancing control device associated withthe grinding wheel, (control unit 200 in FIG. 6).

A random access memory board 254 provides memory for calibration andother routines together with additional memory for use by the centralprocessing units 248 and 250 as required. In particular RAM 254 willinclude the calibration signals derived in relation to the referencestraight edge 224.

The control console 60 shown in FIG. 4 comprises the operator station256 together with terminal and display 258. Typically the terminal anddisplay are incorporated into the console 60.

Programmes for running the machine are entered via the terminal and themachining process and general operation of the machine can be displayedusing both the computer terminal of 258 and other displays associatedwith the operator station 256.

A portable control unit 260 is also shown in FIG. 7. The output from theportable control is fed to the databus via an encoder servo interface262 and/or via line 264 to an input/output unit 266.

The latter also receives data from the wheel balancer 200, the gapsensor 190 and the circumference measuring gauges 234 and 23 all shownon FIG. 6.

The machine services are designated by a signal unit on FIG. 7 at 268.These are described in more detail elsewhere but essentially comprisethe controls for the cooling fluids and the lubricating oils. Data toand from the machine service control is achieved via link 270 via theinterface unit 266. Since a large amount of data may have to betransferred to and from the input/output unit 266 and the databus 246, apair of parallel input/output batches 272 and 274 are provided enablingdata to be transmitted in both directions.

The X-axis encoder 220, Z-axis encoder 180 and the workhead encoder 166provide data to an encoder interface unit 276 and data is supplied fromand to the databus 246 via tne encoder interface unit 276.

Two encoder servo interfaces are provided one denoted by reference 262and the other by reference numeral 278. The former receives data fromthe hand-held unit 260 but also delivers signals to the index drive 204via a digitax drive control unit 280 and to the covers drive 244 via asecond digitax drive control unit 282.

X-axis drive signals are supplied to the motor 176 from an amplifier 284which receives control signals from the second encoder servo interface278.

The wheelhead drive motor 64 is controlled by a power amplifier 286which receives control signals via encoder servo interface 278 andprovides a tacho output from the wheelhead via line 288.

The X-axis drive motor 210 is controlled by a power amplifier 290 whichreceives control signals from a master/slave servo interface 292 alongline 294. The Xaxis tacho 212 provides input signals to the master/slaveservo interface 292 as does also the workhead tacho 164.

Control signals for the workhead drive motor 156 are provided via poweramplifier 296 which is controlled by signals from master/slave servointerface 292 along line 298 and the tailstock drive motor 158 iscontrolled by signals from the power amplifier 300 which itself iscontrolled by signals from the master/slave servo interface 292 via line302. A power supply unit for the amplifier is shown at 304.

Master system control unit 306 receives data from and supplies data tothe bus 246 and receives as input signals outputs from the tailstockload cell 308 the EDW depth gauge 310, swivel lift rectilinearpotentiometer 202, the index drive rectilinear potentiometer 196, thecovers drive potentiometer 248, the shoulder location probe 312, thestraight edge capacitance gauge 228 and the E. loop 314.

A three phase supply and distribution board 316 supplies power to apower supply unit 318 for generating control voltages for the solidstate devices on the various printed circuit boards making up theamplifiers and control systems interfaces, memories and computingdevices shown elsewhere in FIG. 7.

Although the drives are electrically powered and much of the sensing isperformed electrically and electronically, some of the functions on themachine are performed hydraulically.

XIII Hydraulic Circuit

The closed hydraulic circuit is shown in FIG. 8. This comprises a pump320 powered by an AC motor 322 powered from a source 324. A pressurerelief valve 326 diverts surplus oil to a return feed 328 to thehydraulic tank 330 from which oil is drawn by the pump 320 via line 332.A filter 334 protects the pump against the ingress of dirt.

A pressure switch 336 indicates when the line 338 is pressurised and a10 micron filter 340 protects the remainder of the hydraulic circuitfrom particle and foreign matter which might otherwise damage seals etc.

The high pressure flow line is denoted in solid black at 342 andsupplies the various hydraulic facilities to be described.

A first hydraulic drive 344 provides drive to the tailstock to advanceand retract the latter. A load cell 346 senses the thrust exerted on thetailstock by the drive 344 and a load cell signal is supplied to the CNCdisplay along line 348. Flow and control valve means for controlling thetailstock advance drive 344 is designated by reference numeral 350.

The load generated by the tailstock hydraulic cylinder 346 is relayed bythe load cell 348 to the CNC display. The operator may then adjust theload by means of a potentiometer/amplifier/control and a 3-wayproportional pressure control valve, Type 3, DREP 6 (of Rexrothmanufacture). This could be made into a closed loop system. The maximumload may be limited by a disc spring in series with the cylinder/loadcell.

The tailstock is clamped in position by a second hydraulic ram 352 towhich hydraulic oil is supplied under pressure via a non-return valve358 and flow control valve 356. In order to unclamp the valve 356 isaltered to permit through flow to the return path 328 along line 358.

XIV Hydraulic Wheelhead Turntable Lifting

As described in relation to FIG. 6, the grinding wheel head assembly ismounted on a turntable 194 and the turntable is secured into any one ofa large number of different circular orientations each determined by theinter-engagement of cooperating gear teeth, one set on the turntable andthe other fixed in relation to the machine. As previously describedbefore the turntable can be indexed, it must therefore be lifted so thatthe teeth no longer engage.

A hydraulic drive 360 is provided for this purpose and a similar controlto 356 is provided at 362 for supplying oil under pressure from the highpressure manifold 342 via a non-return valve 364, to one side of thepiston in 360 when the table is to be lifted for indexing, and to theother side of the piston in 360 when the table is to be lowered.

XV Hydraulic Actuation of Workpiece Shoulder Measurement

For some types of grinding, a probe is required to determine theposition of shoulders on the workpiece which are to be ground. Ahydraulic rotary actuator for moving the probe arm 366 is supplied withhydraulic fluid via control 368 and a master control valve 370.Operation of 370 lowers the probe-arm and reversing control 370 causesthe probe arm to lift. As with other such control valves a non-returnvalve is provided in the feed line at 372.

XVI Hydraulic Drives for Movomatic Guages

The movamatic gauges 234 and 236 of FIG. 6 must be advanced andretracted and to this end two hydraulic drives are provided for thispurpose denoted by reference numerals 374 and 376 respectively.

Hydraulic control valves 378 and 380 respectively control the flow andreturn of hydraulic oil to the retract and advance drives 374 and 376and a pressure relief valve 382 is provided on the supply line to boththe valves and is preceeded by a variable flow valve 384 so that theflow of oil to the retract and advance drives 374 and 376 iscontrollable and thereby the speed of the gauge slides can be adjusted.

XVII Oil Feed to Hydrostatic Drives

The wheelfeed, lead screw and workslide hydrostatic drives require thecontrolled supply of oil under pressure and FIG. 9 gives details of thesystem supplying the lubricating oil. A holding tank is denoted byreference numeral 386 and an electrostatic cleaning device 388 isassociated therewith. Level sensors 390, 392 and 394 provide oil levelsignals to the control system previously described.

A scavenge pump 396 derives oil from the tank 386 via a non-return valve398 and is driven by a motor 400 derived from a three phase power supply402. A 10 micron filter 404 protects downstream components and a filterblot signal is generated and delivered along line 406 as a warningsignal.

A bleed 408 provides a return path for excess oil to the tank 386 butotherwise the output from the filter serves as the input to a gear pump410 itself driven by a motor 412. A three phase supply 414 providespower to the motor 412 via an inverter drive unit 416. A Eurothermcontroller 418 enables the pressure to be set and a pressure gauge 420indicates the pressure determined by the Eurotherm controller. Thelatter controls the inverter drive 416 and thereby the motor 412.

A reserve accummulator is provided at 422 and the operation of thecontroller and pump is to maintain a constant preselected pressure inthe main supply line 424.

The supply line is fed by the output of the gear pump 410 via a secondfilter 424 from which a filter blot signal can be derived along line 426when appropriate. The temperature of the oil is controlled by arefrigeration unit 428 and the oil temperature downstream of the unit428 is sensed by a platinum resistance probe 430 which in turn controlsa temperature controller 432 which dictates whether or not therefrigeration unit 428 is to function and if so to what extent. Anon-return valve 434 feeds the main line 424.

XVIII Workslide Bearings

The workslide bearings are fed via a heater 436 and further filter 438via line 440. The filter blot signal is derived along line 442.Workslide bearings are denoted by reference numeral 444 and thetemperature of the oil leaving the bearing is sensed by a resistanceprobe 446 the output of which controls a temperature controller 448which in turn controls the degree of heating imparted by the heater 436to the oil flowing therethrough. The temperature controller and heaterderive power from a three phase supply 450. A pressure sensitive switch452 indicates under or over pressure in line 440 and a return line ordrain 454 returns used oil to a return manifold 456 to a gravitycollection tray 458 which feeds and returns oil to the holding tank 386.

XIX Wheelfeed and Leadscrew Bearings

The wheelfeed bearings 460 and the leadscrew bearing 462 are eachsupplied with oil via a feedline 464 from a second heater 466 and filter468 from which a further filter blot signal can be derived along line470. The temperature of the wheelfeed bearing is sensed by a resistanceprobe 472 which in turn controls via temperature controller 474 theextent of heating imparted by the heater 466. As before power for theheater and controller is derived by a three phase supply 476.

As before pressure sensitive switches generating under or over pressuresignals are provided at 478 and 480 respectively. Drains are provided at482 and 484 for communicating with the return manifold 456.

XX Oil Supply to Rotational Bearings

The rotating bearing associated with the wheelhead, workhead andtailstock also require a supply of oil under controlled temperature andpressure and this is shown in FIG. 10. In this Figure the oil supplysystem contained within the dotted outline 486 may be the same oilsupply system as employed for the linear bearings or may be a separateidentical supply system dedicated to supplying oil at appropriatepressure for the rotational bearings. In either event the systemoperates generally in the same way as that described in relation to FIG.9.

The supply system provides oil under controlled pressure to the mainfeed line 488 (which corresponds to the feedline 424 in FIG. 9) and theoil is distributed to the wheelhead, workhead and tailstock bearingassemblies via three heaters 490, 492 and 494 respectively. In eachfeedline a filter 496, 498 and 500 is provided as a further protectionfor each of the bearings and the pressure of the oil supply to thebearing is sensed by a pressure sensing switch 502, 504 and 506respectively. The wheelhead bearing is shown at 508, the workheadbearing at 510 and the tailstock bearing at 512.

XXI Temperature Control of Oil for Bearings

The temperature of the oil supplied to each of the bearings iscontrolled by the heaters and the temperature of each of the bearings issensed by platinum resistance probes 514, 516 and 518 respectively whichin turn control the heaters via temperature controllers 520, 522 and524. Three phase power supply for each of the heaters is provided at526, 528 and 530.

XXII Workpiece Cooling

The workpiece is cooled in known manner by supplying cutting oil or anemulsion of oil and water under pressure thereto. The temperature of thecutting oil is controlled and the supply and control elements are shownin FIG. 11. To this end a holding tank is denoted by reference numeral532 with level sensors 534, 536 and 538 respectively for sending levelsignals to the control system previously mentioned. Cutting oil is drawnfrom the tans 532 via non-return valve 540 by a pump 542 driven by amotor 544 itself powered by a three phase 546. A pressure relief valve548 returns unwanted oil to the holding tank 532 via line 550 therebymaintaining the pressure in the supply line 552 at the pressure set bythe relief valve 548.

Multi-element profile vessel filters are provided at 554 and the oil isthen supplied first to a refrigeration unit 556 the output temperatureof which is sensed by a temperature resistance probe 558 feeding atemperature controlling device 560 for controlling the degree ofrefrigeration and maintaining the temperature of the oil in the line 562constant. The now cooled oil is then heated by heater 564 itselfcontrolled by a temperature controller 566 which in turn receives afeedback signal from a temperature probe which may be remote from theheater 564 and is sensing the temperature of the oil shortly before itis applied to the workpiece. The wheel 28 and workpiece 30 are shown inthe grinding position and a coolant supply feed 570 directs cooling oilonto the workpiece-wheel engaging region and is supplied with oil via acontrol valve 572. A transducer associated therewith provides a returnsignal to the control system indicating whether coolant is on or offalong line 574.

Cooling oil not required is returned via path 576 to a gravitycollection tray 580 for returning oil to the holding tank 532.

In the same way coolant which has been applied to the workpiece/wheelinterface drain as shown by the dotted line 582 also into the gravitycollection tray.

Coolant oil for the electrostatic dressing of the wheel is suppliedalong line 584 from a second control valve 586 which also includes atransducer 588 for indicating when the control is on or off. As beforeunwanted oil is returned via path 59C to the gravity collection tank viapath 576.

XXIII Different Grinding Processes

FIGS. 12 to 16 inclusive illustrate how the wheel must be formed andangled so as to perform specific types of grinding.

In FIG. 12 wheel forming by 29 provides a 5° relief angle on oppositecircumferential edges of the wheel 28 and the wheelhead table is indexedso as to present the wheel on either side of a central flange which isto be ground with a 2½° clearance angle.

Infeed movement is denoted at 592 to perform cylindrical grinding and at594 to perform radial grinding of the shoulders of the flange.

The control of rotation of each of the headstock and tailstock is suchthat the tailstock is driven by the same demand as the headstock inorder to provide equal torques in both units.

The wheel may be conditioned periodically by an electrolytic wheeldressing system at 31.

The workhead and tailstock, worktable and wheelhead drives may be linkedand synchronised to permit complex grinding to be performed usingelectronic control and feedback such as described in UK Patent 1331601.

FIG. 13 indicates how a tapered component can be shaped by electrolyticconditioning at 31 to escape the abrasive grit and forming at 29 theoutside surface of the wheel 28 with a gradient and a chamfer around onecircumferential edge thereof. Operating wheelfeed in the manner shown at596 and retracting the wheel as it moves along the Z-axis from the righthand end to the left hand end of the workpiece produces a taperingdiameter.

FIG. 14 indicates how the edge of the wheel can be electrolyticallyconditioned at 31 and formed by dressing wheel 29 to produce a smoothcurve across the thickness of the wheel 28, using a specially shapedelectrode 31. X-axis infeed movement is shown at 598. An alternativefinished form is shown in chair dotted outline at 600. Any shape can beformed along the length of the workpiece 30 by appropriate control ofthe infeed during the Z axis traverse.

In FIG. 15 the wheel is electrolytically dressed at 31 and formed bywheel 29 so as to have tapers (ie small radius corners) before it isadvanced into the narrow gap designated 602 and traversed from oneshoulder 604 to the other 606 to finish grind the reduced diametercylindrical section 608.

A workpiece comprising a shaft with steep tapers is shown at 33 in FIG.16. The wheel is electrolytically dressed at 31 and formed by 29 so asto have a lead angle and radius as shown at FIG. 16a and movement of thewheelfeed to achieve the different sections of the workpiece are denotedat 610, 612 and 614 respectively, those at 610 and 612 being effectedeither by tilting the wheel or by appropriate relative movement of theworktable and wheelfeed to obtain the effective traverses shown at 610,612.

XXIV X and Z Axis Measurement

As already mentioned, errors due to distortion, misalignment andparallax are avoided by mounting all the measuring systems in the sameplane as far as possible and in linear alignment with the maindistorting mechanism, ie the grinding wheel.

To this end the wheelslide X-axis encoder scale is mounted at the heightof the wheel axis and in line with the face of the wheel so that itsassociated reading head, which is located in close proximity to thesurface of the scale, will itself be located at the same height and inthe same vertical plane as the wheel.

X-axis correction is achieved by checking for changes in capacitance ofa capacitance gauge formed by a conductive straight edge (mounted on theworktable in the opposite side of the latter from the grinding wheel)and a fixed conductive probe mounted for rigidity to the machine bed.The conductive strip on the face of the straight edge is itself arrangedto be in the same horizontal plane as the wheel axis, so that the probeis at the same height, and the probe is laterally fixed in position inthe vertical plane containing the grinding wheel so as to be coplanarwith the plane containing the X-axis encoder behind the wheel as well asat the same height as the linear scale of the X-axis encoder.

By mounting the workpiece about an axis for rotation which is parallelto the wheel axis and the Z-axis (the linear direction of worktablemovement) and at the same height as the wheel axis, the point ofengagement between wheel and workpiece should be at the same height sothat it, the wheel axis, the X-axis encoder scale, the conductiveelements forming the X-axis connection capacitance probe and theworkpiece axis are all in the same horizontal plane.

The only measuring device not in the same horizontal plane is the Z-axisreading head 180, but as mentioned previously, by mounting this in thesame vertical plane as that containing the wheel, albeit below thehorizontal plane containing the wheel axis, any errors arising from thisdisplacement appear not to affect the accuracy of the machinerycapabilities of the machine, and acuracies in the nanometric range havebeen achieved during preliminary trials of the machine.

Constructional Detail of Overall Machine

FIG. 17 shows the machine in the fully enclosed state with all covers inplace. Windows at 616 and 618 permit operation of the grinding processto be observed. Fine adjustment to the operation of the machine can beeffected by adjusting the controls on the control console 620.Workpieces waiting to be ground can be stored within the cabinetssurrounding the machine, typically within that marked 622, for whichpurpose a door (not shown) may be provided at the end of the housing. Inthis way workpieces awaiting machining can be acclimatised to theoperating temperature of the machine so as to reduce thermal shockand/or thermal distortion.

With the covers removed, as in FIG. 18, the various component parts ofthe machine can be seen. Thus the polymer concrete frame 20 can be seensupported by the intermediate base 14 and the latter by means ofvibration isolating and levelling feet two of which are visible at18,48. The grinding wheel is just visible at 28 as is the forming wheelat 29, at the lower end of the housing 624 containing the forming wheeladvance/retract mechanism.

Coolant is sprayed onto the grinding wheel via a nozzle 626 and thewheel guard is adjustable at 628 and includes an electrolytic wheeldressing device 630 for periodically dressing the grinding surface ofthe wheel 28.

A fixed worktable cover at 632 obscures the straight edge capacitanceprobes and 2 axis grating. Telescopy shrouds at 634 and 636 protect theslideways on which the worktable, headstock and tailstock run. Theworktable is just visible at 638.

Worktable

Turning now to the more detailed drawings, FIGS. 19, 20 and 21 revealdetail of the construction and support of the worktable. This comprisesa metal casting 638 having a flat underside machined to run on twoceramic slideways 640, 642 each mounted on the fixed machine frame 20.Sliding faces are formed in the casting at 644, 646, 648 and 650 toengage slideway 640 which sliding faces are formed in the casting onlyon two faces around 642 namely at 652 and 654.

The underside of the casting is cutaway generally centrally over itsentire length at 656 to accommodate one of the drives to be described.

A machined flat upper surface of the casting at 658 supports an elongateceramic block 660 forming a straight edge and a reading head 662including a capacitance probe 664, is mounted firmly to the machineframe bed 20.

The slideways 640, 642 are shown in FIG. 21 as being mounted at the topof upright legs 666, 668 respectively, also firmly attached to themachine frame/bed 20.

A running clearance of the order of 0.035 mm is provided between allbearing surfaces such as between 648 and 640.

Linear hydrostatic bearings are formed at the bearing surfaces byproviding drillings such as 670, 672 for supplying fluid, typically alubricating oil, under pressure, so as to form a pressurised oil flowbetween each of the pairs of sliding surfaces. Similar drillings supplyoil to all the other bearing surfaces, so that when operating, the metalsurfaces such as 648 are separated from the faces of the ceramicslideways by a very thin film of oil. Galleries and manifolds such as674, 676 serve to supply oil under pressure to all the bearing surfaces.

The face of the worktable on which the headstock and tailstock are to bemounted is inclined at approximately 450 and is cut away at regionsalong its length as at 678 to provide an overhanging shoulder 680 belowwhich a protruding part of the device to be mounted on the slidewayformed by the inclined surface can be fitted and clamped, to secure thedevice in position as required.

A large diameter hole 682 extends through the entire length of thecasting 638 to allow services to traverse from one end of the otherunimpeded and to permit a rigid elongate link to be established betweendevices mounted at opposite ends of the worktable (to be described).

The tailstock is shown in outline in FIG. 19 at 684 and the headstock inFIG. 20 at 686. Associated with the latter is the wheel formingmechanism housing 624 (see FIG. 18). The grinding wheel 28 and formingwheel 29 are both shown in FIG. 20.

FIG. 22 shows detail of the measuring unit mounted on the flat surface658 (see FIG. 20). This comprises a base 688 providing a flat supportsurface 690 for a bearing plate 692 one edge of which is upturnedthrough 900 to provide one lateral support 694 for an accuratelymachined rectilinear block of ceramic material 696 which is carried onrolling bearing 698, 700 and is clamped at intervals along its length byT pieces, one of which is shown at 700. Leaf springs as at 702, 704force the block 696 in a downward sense.

Vertically mounted rollers, one of which is denoted by 706 are carriedby an upturned section 707 of the base 688 at similar intervals alongthe length of the blocks 696, and leaf springs at 708 in the upstandingedge 694 of the plate 692.

In front of the worktable 638 is mounted the reading head housing 662which inter alia carries the probe 664 carrying at its end nearest theblock 660 an electrode 710. The face 696 of the block 660 is metallisedusing a hard chrome or the like, in the region which is traversed by theprobe electrode 710, to form a spaced apart elongate electrode 714,which together with the probe electrode 710 comprises a capacitance thevalue of which will vary if the spacing between the electrode 714 on theblock 660 and the electrode 710, alters due to distortion of themachine.

Since the electrode 710 is fixed to the machine frame 20, it can beassumed to be stationary and as previously described, an error signalfor calibrating the grinding wheel feed can be stored for

all positions of the table 638.

Also on the base 688 is mounted an elongate grating 712 and the housing662 includes an optical sensor 720 for detecting the grating divisionsand generating electrical pulses in known manner, so that the Z axisposition of the table 638 can be determined by counting the pulses (inknown manner) as the table moves from left to right and vice versa.

The ceramic block 660 which constitutes the straight edge and thegrating 712 can also be seen in FIGS. 23A, B and 24A, B.

The elongate strip of metallising 714 on the face 696 of the block 660is earthed by earthing strip 716.

The housing 622 is shown in FIG. 23A as comprising two uprights and across piece 718 on which the probe 664 is mounted. Below can be seen theoptical reading head 720 which cooperates with the grating 712 to givethe Z axis measurement.

A T-piece support and associated lateral supports such as shown at 700,694 and 707 are in fact provided at two points one on each side of thehousing 662. The second assembly is denoted by reference numeral 722.

The ceramic block 660 is located at the left hand end by a button 724and at its right hand end by two leaf springs 726, 728. The button isthreadedly engaged in its suport 730 and can be screwed in to preloadthe block 660.

The grating is screwed to the base 688 at intervals along its length asat 732, 734.

As previously described a second electrode is provided on the housing662 to form a capacitance bridge. This can be seen at 736 mounted atright angles to the first electrode 710. An electrode 738 formed on theface of a block 739 as by metallising, and spaced from 736 by the samemean distance that electrode 710 is spaced from the strip 714, providesthe second electrode of the second capacitor.

Limit switches are shown at 740, 742 and adjustable stops extend to theleft and right of a platform 744 at 746, 748, to engage end stops 750,752 respectively (see FIGS. 23A, B).

The position for the housing 662 is selected so as to be generallyopposite the grinding wheel 78.

The ceramic block 660 can be adjusted to align it with the worktabletrajectory by a sprung arm 754 (see FIG. 24B). This is anchored at 756and includes a containing region at 758 to allow the remainder of thearm to distort relative to the fixed end at 758. The roller 760 is letinto a groove 762 in the side face of the arm 754 and the free end ofthe arm is held captive by means of a bolt 764 having a compressionspring 766 trapped between the enlarged head of the bolt 764 and theseating 768. Arm stop is provided by a screw 770 which is rotatable toadjust the pointed end so as to force the arm more or less towards theblock 660.

The roller 760 is sandwiched between the arm and the block 660, so thatscrewing in the screw 770 will tend to push the block 660 against thespring 708 and compress the latter.

FIG. 25 shows the detail of the adjusting screw 770.

Worktable Drive

The worktable is moveable from left to right and vice versa by afriction drive created by a pair of grooved rollers 778, 780 mountednear the lower end of a drive unit housing 783 (see FIGS. 26, 27). Theunit 783 is bolted to the left hand end of the worktable 638.

The rollers 778, 780 nip a circular section rod 782 which extendsbetween the right hand end 784 of the machine frame 20 and a bracket 786mounted on the frame towards the opposite end thereof.

The rod is tensioned and preloaded by a spring 788 (which may be made upof bevelled washers) and a bolt 790 which is threadedly engaged in theend of the rod 782 and whose enlarged head acts on the spring 788 via athrust washer 792.

At the left hand end the rod is held in place by a pair of metal striphinges 794, 796 pinned to the rod at 798 and to a bracket 800 at 802.

Supports 804 for the rod to reduce bending and droop are located atregular intervals along the length of the rod 782. Each includes aspring loaded plunger 806.

The measuring unit is protected by a cover shown in FIG. 28. Seals areprovided at 808, 810, 812 to reduce the ingress of dirt via the gapsleft to accommodate the legs 814, 816 (see FIG. 22) which support theplatform 744 (see FIG. 23A).

Slideway Covers

FIGS. 29 to 37 contain detail of the telescoping covers mounted atopposite ends of the worktable and which are designated 634 and 636 inFIG. 18.

FIG. 29 shows the left hand end of the machine and the set oftelescoping shrouds 636. The extreme left hand shroud is bolted to anend of the machine frame generally designated 818 and the right hand endof the telescoping set terminates in a bulkhead plate 820. Each of theshrouds includes a grooved wheel such as 822 which runs on a rod 824which runs parallel to the bar 782 which provides the rail for thefriction drive for the worktable made up of the two rollers one of whichis shown at 780 in FIG. 29.

The second rail for the cover wheels can be seen in FIG. 30 at 826.Shown in dotted outline is the opposite wheel for the right hand shrouddenoted by reference numeral 828.

The drive unit for the friction drive made up of the wheels 778, 780 isdenoted by reference numeral 783 and the telescoping shroud is formedwith an extension housing 830 to accommodate the additional height ofthe friction drive 783.

Also visible in FIGS. 29 and 30 is the large diameter rigid tube 832which serves to connect the left hand bulkhead plate 820 to thecorresponding right hand bulkhead plate 834 (see FIG. 32). Detail of themethod by which the ends of the tube 832 are secured is shown in FIG. 37in which the tube 832 is shown as having an end of reduced diameter witha flange which can be bolted to the bulkhead plate 834 by means of nutssuch as 836.

Drive for the covers is provided at the right hand end of the machineand detail of this is contained in FIG. 32.

Drive is transmitted via a threaded rod 838 which is engaged in a nutheld captive in an assembly generally designated 840. The nut ispreloaded by means of a spring 842.

Rotation of the threaded rod 838 is achieved by an electric motor 844the outward end of which is connected to the rod 838 via a coupling 846.The unthreaded end of the rod 838 runs in a bearing 848 and the couplingand bearing are contained within a housing generally designated 850 towhich the motor 844 is attached.

As will be seen from FIG. 34, the rod 838 extends below the worktableand because of the alignment of the rod, the worktable is cut away aspreviously described in connection with FIGS. 19 and 20 in the regiondefined by reference numeral 656 to accommodate not only the rod butalso an end bracket 852, which is secured to the frame of the machine20. The rod 838 extends through the bracket and is secured in place bymeans of a nut 853.

The rod 782 for the worktable friction drive and the rod 838 extendthrough the machine substantially coaxially but since the rod 838 isengaged by a plate located at the right hand end of worktable and thefriction drive for the worktable is mounted at the left hand endthereof, the rod 782 for the worktable friction drive does not need toextend any further across the worktable than is sufficient to enable afull traverse of the worktable to the right hand position of itstraverse and likewise the rod 838 does not need to extend any more underthe worktable than is sufficient to permit the right hand end of theworktable to move to its extreme right hand position. Both rods 838 and782 therefore can extend below the worktable and terminate withoutinterfering the one with the other.

FIG. 33 also illustrates the covers which are attached to the front andback of the worktable 638, namely cover 854 which is bolted to thevertical face 856 of the worktable 638 and the rear cover 858 bolted tothe inclined face of the worktable 638. These two covers 854 and 858extend over the length of the worktable and serve to protect the twoslideways 640 and 642.

FIG. 31 shows in more detail the mounting of the wheels such as 822 and828 and also shows the hollow box trunking 860 which protrudes to theside of the shroud 862 and provides a housing for services such as pipesand cables, one of which is designated by reference numeral 864.Mounting brackets for securing to such services are shown at 866 and868. A drag chain is secured to the open end of the trunking 860 asdenoted by reference numeral 636 and the opposite end of the drag chainis attached to the base of the machine as previously referred to withreference to FIG. 18.

The services are contained within the drag chain and are of flexiblenature so that as the drag chain varies in shape the services followsuit.

The position of the covers drive in relation to the right hand end ofthe machine is best seen with reference to FIG. 35. The motor 844includes a mounting flange 870 by which it can be bolted to the housing850 (see FIG. 32).

The precise position of the covers is determined by means of a linearpotentiometer 872 (see FIG. 36) one end of which is anchored in abracket 874 attached to the bulkhead place 834 via bracket 875 and theother end of which is attached via a stud 876 to the right hand end ofthe worktable 638.

A clearance hole 878 in the right hand bulkhead plate 834 allows thestud 876 to pass through to the worktable 638.

The potentiometer is therefore able to measure the gap between theworktable 638 and the bulhead place 874 so the latter (and the guardattched to it) can be slaved to the worktable.

As with the left hand end, the extreme right hand end guard shroudincludes a solid end 880 which is similarly bolted to an upstandingflange 882 forming part of the extreme right hand end of the machineframe.

In order to provide for peripheral sealing to the shrounds, an endplateis bolted to each end of the worktable 638. The plate at the left handend is shown in FIG. 29 at 884 whilst the plate bolted to the right handend is visible in FIG. 36 and is denoted by reference numeral 886. It isalso visible in FIG. 32.

As best seen in FIG. 29, the plate 884 is formed with a gutter 888 whichcooperates with a downturned peripheral lip 890 secured around theperipheral edge of the plate 820.

A similar arrangement is provided at the other end of the worktable sothat the two peripheral regions of the plates 886 and 834 are similarlysealed.

The engagement of the downturned lip 890 with the trough of the gutter888 acts as a good seal against moisture ingress and as shown in FIG.34, a further sealing can be effected using a strip of rubber or rubberand plastics composite material 892 secured to the plate 834 so as tosurround the engagement between the inturned lip and the gutter. Asimilar ring of material may be used at the opposite end of theworktable which is not shown in so much detail in FIG. 29.

Headstock

FIGS. 38 and 39 should be read together. FIG. 38 shows the inboard endof the headstock whilst FIG. 39 shows the rear extension to theheadstock housing for the motor resolver and other components parts fortating the headstock.

In conventional manner, a workpiece is mounted between two stocks, oneon the headstock and one on the tailstock. The headstock mounting isdenoted by reference numeral 894 and this is keyed into an end of ashaft 896. The latter includes an annular shoulder 898 and extends withprogressively reduced diameter to form a motor shaft for a direct driveelectric motor having a stationary winding 900 and a rotor 902 which isattached to the shaft 896.

The shaft is supported in hydrostatic bearings generally designated 904and 906 and the large diameter annular region 898 provides two shouldersfor creating thrust bearings at 908 and 910 respectively. Oil forfeeding the bearings is supplied via ports and drillings (not shown inFIG. 38) so as to occupy the space between the shaft and the internalsurfaces of the bounding sleeve 912 in the case of bearing 904 and 914in the case of bearing 906. Oil which escapes axially during use,returns via ports and drilling such as 916, 918 and 920 to return to thesump.

In this connection the temperature of the returning oil is detected by atemperature probe 922, electrical signals from gutter 888 acts as a goodseal against moisture ingress and as shown in FIG. 34, a further sealingcan be effected using a strip of rubber or rubber and plastics compositematerial 892 secured to the plate 834 so as to surround the engagementbetween the inturned lip and the gutter. A similar ring of material maybe used at the opposite end of the worktable which is not shown in somuch detail in FIG. 29.

Headstock

FIGS. 38 and 39 should be read together. FIG. 38 shows the inboard endof the headstock whilst FIG. 39 shows the rear extension to theheadstock housing for the motor resolver and other components parts fortating the headstock.

In conventional manner, a workpiece is mounted between two stocks, oneon the headstock and one on the tailstock. The headstock mounting isdenoted by reference numeral 894 and this is keyed into an end of ashaft 896. The latter includes an annular shoulder 898 and extends withprogressively reduced diameter to form a motor shaft for a direct driveelectric motor having a stationary winding 900 and a rotor 902 which isattached to the shaft 896.

The shaft is supported in hydrostatic bearings generally designated 904and 906 and the large diameter annular region 898 provides two shouldersfor creating thrust bearings at 908 and 910 respectively. Oil forfeeding the bearings is supplied via ports and drillings (not shown inFIG. 38) so as to occupy the space between the shaft and the internalsurfaces of the bounding sleeve 912 in the case of bearing 904 and 914in the case of bearing 906. Oil which escapes axially during use,returns via ports and drilling such as 916, 918 and 920 to return to thesump.

In this connection the temperature of the returning oil is detected by atemperature probe 922, electrical signals from performed as frequentlyas is required to keep the wheel true.

By utilising movement along both X and Y axes, so complex profiles canbe dressed onto the grinding wheel.

FIG. 40 shows in cross-section the hydrostatic bearing 904 of FIG. 38.The shaft 896 is a running fit within the six flattened ridges one ofwhich is denoted by reference numeral 956 and oil is supplied to the sixequal equidistant arcuate regions between the ridges 956, 958 etc suchas the region 960, by means of ports and drillings which is designatedby 962. Oil under pressure is supplied to a manifold drilling 964 via aninlet 966 and journal restrictors such as 968 within the drilling areprovided to control the final pressure of the oil supplied to theannular regions such as 960.

Each annular region also communicates via a second drilling (970 in thecase of annular region 960) with a port shown in dotted outline at 972in the case of drilling 970. Normally these ports such as 972 areblanked off but if, as is desirable, the individual pressures within thedifferent regions such as 960 is to be monitored, the blanking may beremoved and pipe connectors such as 973 may be inserted and pipes suchas 974 used to join the connectors to pressure transducers such as 976.

Each transducer 976 may be responsive solely to the pressure from theoil in one pipe such as 974 in which case an absolute output signal willbe obtained therefrom along line 978 indicative of the actual pressureof the oil within the space communicating with that port. In the case of973, this is the annular space 980.

The pressure transducer 976 may alternatively comprise a differentialdevice (as shown) and in that event oil pressure from the oppositeannular region (denoted by reference numeral 982 in the case of region980) is supplied via drillings such as 984, 986, pipe connector 988 andpipe 990 to the opposite side of the differential transducer as shown.The output signal from such a transducer, along line 978, will nowrepresent any difference between the pressure in the region 980 and theregion 982 and the “sign” of the signal, (which indicates whether thepressure in 980 is greater than that in 982 or vice versa), can be usedto denote which side of the bearing is being subjected to the greaterpressure at any instant. The amplitude of the differential pressuresignal indicates the level of the force which may not be merely an outof balance force but also grinding and other external forces.

A feedback signal from a series of such transducers can be generated inthe manner already described herein so as to provide part of the controlsignal for a worksteady control system (as will be described withreference to FIG. 70), to compensate for any out of balance forces inthe bearing which could result in a circular surface being ground whichis less than true. The other control signals are obtained fromtransducers associated with the tailstock to be described.

FIG. 41 is an external view of the right hand end of the headstock shownin FIG. 38 showing the various ports (but without the pipe connectorssuch as 972 in place). A dressing wheel is shown at 952 and theheadstock at 894.

Tailstock

FIGS. 42 to 44 give detail of the tailstock assembly. As with theheadstock, a workpiece is supported by the stock 992 which is securedinto the left hand end of the main shaft 994 of the tailstock assembly.In this connection FIGS. 42 and 44 must be read together since 44contains the right hand end detail of the tailstock assembly.

As with the headstock shaft, shaft 994 is of progressively reducingdiameter from the stock end to the motor end. Midway the shaft includesa radially enlarged annular region 996 and thrust bearings are formed oneither side of the annular region 996.

Two hydrostatic bearings are formed along the length of the housingcontaining the section of the shaft 994 to the left of the annular ring996 one denoted by reference numeral 998 and the other by referencenumeral 1000.

Similar hydrostatic bearings are formed on either side of the radialfaces of the annular flange 996 as denoted by reference numerals 1002and 1004. Oil is supplied to the bearing faces in manner know per se bymeans of drillings and ports such as are shown in FIG. 44 but not shownin FIGS. 42 and 43. Oil which leaves the bearing regions collects in theannular reservoirs such as 1006, 1008, 1010 and 1012. Drillings such as1014, 1016m, 1018 and 1020 convey the oil back to a main reservoir.

The temperature of returning oil is determined by a temperature probe1022 electrical signals from which are supplied to the control systempreviously described.

The shaft extension beyond the annular flange 996 carries a rotor 1024.The stator windings 1026 are cooled by passing a cooling fluid around ahelical path formed by a helical thread profile 1028 around the statorand closed by an annular sleeve 1030 in the same way as the motor in theheadstock assembly is cooled.

The brushless motor needs a commutator and this is provided by aresolver 1032 mounted on a shaft extension 1034 the end of which isearthed via earthing brushes, one of which is shown at 1036.

In order to enable mounting and demounting of a workpiece between theheadstock and tailstock, it is normal practice to arrange for thetailstock to be retractable and to this end the tailstock assembly shownin FIGS. 42 and 44 is retractable by 32 mm from the position shownthrough the distance denoted by reference numeral 1038 in FIG. 42. Tothis end the housing section 1040 containing the motor is movableaxially relative to the housing section 1042 containing the hydrostaticbearings 998 and 1000. The housing 1040 is secured to a flanged bracketgenerally designated 1044 with the flange being secured to the left handend of the motor housing between the latter and the housing sectionscontaining and defining the thrust bearing arrangement around theannular flange 996. To the left hand end of this a cylindrical sleeveextends at 1046 to provide a cylinder within which the inner cylindricalbearing member 1048 can slide in the manner of a piston and a seal 1050is provided between the bearing member 1048 and the cylindricalextension 1046 so that oil in the annular reservoir 1010 returning tothe main reservoir will not leak. By the same token dirt and moisture isprevented from entering.

The oil return path 1016 needs to communicate with the thrust bearingand to this end a sleeved joint is provided formed from the cylindricalsleeve 1052 sealed within the drilling 1016 by means of the seal 1054and secured at its right hand end to the cylindrical housing generallydesignated 1056 within which the thrust bearing assembly is located.Drillings such as 1058 within the housing 1056 allow oil from the thrustbearing region to return via the hollow sleeve 1052 to the drilling 1016irrespective of the position of the sleeve 1052 relative to the seal1054.

Relative movement of the tailstock assembly is achieved by means of ahydraulic or pneumatic cylinder which is conveniently mounted on theunderside of the tailstock casting as shown in FIGS. 43A and 43B.

The pneumatic or hydraulic cylinder is denoted by reference numeral 1062and this is secured at one end via a pin 1064 and link 1066 to a crossmember 1068 of the tailstock casting, generally designated 1070 and theother end of the cylinder is attached to the downwardly extendingsection 1060 of the flange part of the flange bracket 1044 of FIG. 42.

The cylinder is shown in dotted outline in FIG. 43B at 1062 and showshow this and the flange extension 1060 are accommodated within a cut-outregion in the casting which is adapted at 1072 to fit over the upper endof the worktable platform. To this end wearing surfaces are mounted at1074 and 1076 and also at 1078 which engages the lower sliding surfaceof the slideway of the worktable.

As shown in FIG. 42, the cylinder is in its extended mode so that thetailstock 992 is in its advance workpiece engaging mode.

Retracting the cylinder 1062 retracts the shaft 994 and therefore thetallstock 992 releasing the latter from engagement with the workpieceand allowing the latter to be demounted and replaced by a freshworkpiece for grinding.

Although not shown, the cylinder 1062 may be mounted on the axis of theshaft 994 for greater accuracy. Axial mounting will reduce any tendencyfor tilting which may arise with the cylinder mounted on the undersideof the assembly and acting off centre as shown.

FIG. 44 shows the drillings and ports which supply oil under pressure tothe six hydrostatic bearing regions around the shaft 994. One suchdrilling is denoted by reference numeral 1080 and a second drillingshown in dotted outline at 1082 communicates with the hydrostatic padfor sensing the pressure of oil in the pad.

A similar pair of drillings communicates with each of the pads aroundthe shaft 994 as shown in FIG. 44. Pressure transducers such as 1082commuted to the ports (in the same way as is described in relation tothe headstock hydrostatic bearings) provide signals relating to thedisolute pressures in the different pads or the differential pressure sbetween diametrically opposed pads such as 1083, 1085 in FIG. 44. Inthis way any out of balance forces in the tailstock hydrostatic bearingcan be detected and signals relating thereto (eg from line 1087) can becombined with signals derived from the hydrostatic bearings in theheadstock (eg from line 978) to generate a correcting force to beapplied to the workpiece via a moveable (ie active) workrest (notshown).

Wheelhead

FIGS. 45 to 47 reveal detail of the platform on which the grinding wheeland drive is to be mounted and which allows the wheel to be advanced andretracted towards and away from the workpiece held between the headstockand tailstock of the worktable previously described.

Grinding wheel 28 is shown in chain dotted outline at the upper end ofFIG. 45 which is a cross-section through the table as viewed from theworkpiece.

Sliding is effected by mounting the table on two ceramic rails 1082 and1084 which themselves are carried at the upper ends of elongate struts1086, 1088 firmly attached at their lower ends to the machine frame 20.

The platform shown in cross-section is generally designated by referencenumeral 1090 and is machined on its underside so as to providehydrostatic pads at 1092 and 1094 in the case of rail 1082 and 1096,1098, 1100 and 1102 in the case of rail 1084.

Drillings such as 1004, 1006 provide oil to the hydrostatic pads 1094and 1092 while similar drillings 1008, 1110, 1112 and 1114 provide oilto the pads 1096, 1098, 1100, 1102 respectively.

A wheelhead spindle assembly generally designated 1116 is mounted forrotation about a vertical axis 1118 and a wheelhead lift and turnassembly generally designated 1120 is mounted within an aperture in thecentre of the casting 1090.

The purpose and operation of the latter will be described with referenceto later drawings.

Movement of the table 1090 along the rails 1082, 1084 is effected byrotation of a threaded rod 1122 (see FIG. 46) which runs in a nutassembly on the underside of the table so that rotation of the threadedrod 1122 reflects longitudinal movement of the table 1090. The threadedrod and cooperating nut are formed as a hydrostatic screw.

Also mounted on the table is a measuring system including a grating (tobe described) generally designated 1124. The measuring device provideselectrical signals indicative of the position of the table relative to ahome position so as to allow controlled advance and retraction of thegrinding wheel 28.

Services for the wheelhead spindle drive, and other drives on the tableare provided via a drag chain 1126 one end of which is attached to thespindle housing and the other end of which is secured to the wheelheadtble 1090. The second drag chain (42 in FIG. 2) carries services fromthe wheelhead table 1090 to the bed 20.

The spindle drive motor housing 1128 extends on one side of a housinggenerally designated 1130 on the other end of which protrudes a shaft onwhich the grinding wheel 28 is mounted.

The housing 1130 is mounted on a generally circular support which isrotatable about the central axis 1118 (see FIG. 45), to allow the angleat which the wheel 28 is presented to the workpiece, to be altered. Thecircular base is denoted by reference numeral 1132 and an actuator 1134pivotally mounted in a bracket at 1136 acts through a rod 1138 onto abracket 1140 so that extension of the actuator produces rotation in aclockwise sense and retraction of the actuator rotation in an oppositesense about the central axis 1118.

An actuator drive typically in the form an electric motor is denoted byreference numeral 1142.

Oil feed to the hydrostatic screw is effected through union 1144.

A cross-section on the line AA of FIG. 46 is shown in FIG. 47. Thespindle drive motor is typically electrically powered but withhydrostatic bearings and hot oil from the latter must be collected to acentral sump. Since the wheelhead spindle motor 1128 is mounted on arotatable platform, a mechanism must be provided by which hot oil canreturn to the sump. This is shown by a pivoting oil duct 1146 whichterminates in a top hat rotating seal assembly 1148 which is shown incross-section in FIG. 47. Hot oil returning along 1146 flows down thecentral tube 1150 and is directed to the central sump by a drain tube1152.

The entry port for oil to the hydrostatic bearing shown diagrammaticallyat 1152 is denoted by reference numeral 1154.

Wheelhead Feed

FIGS. 48 to 51 illustrate the hydrostatic drive for advancing andretracting the wheelhead table 1090 of FIG. 45. FIGS. 48 and 49 shouldbe read together since 49 is a continuation to the right hand side ofthe assembly shown in FIG. 48. Intermediate the extreme ends is aplatform generally designated 1156 on which the table 1090 is fitted.The table 1156 is shown in dotted outline in FIG. 45.

The right hand end of the lead screw is held captive in a hydrostaticbearing assembly generally designated 1158 itself mounted on the machineframe 20. Oil for the hydrostatic bearing is supplied via pipes to andfrom a union generally designated 1160.

The threaded section of the lead screw is denoted by reference numeral1162 and over its exposed length the lead screw is protected by atelescoping cover generally designated 1164 on the right hand side ofthe platform 1156 and by a similar telescoping cover 1166 on the lefthand side of the platform 1156.

The lead screw runs in a hydrostatic nut below the platform 1156.

The drive for the lead screw is a brushless electric motor generallydesignated 1168 the rotor 1170 of which is axially clamped to the shaft1175 and the stator windings and stator of which is generally designated1172 are cooled using a helical passage for cooling water or oil 1174 aspreviously described in relation to the other electrically poweredhydrostatic drives.

The shaft 1175 is supported in a hydrostatic journal bearing at 1176 andincludes an enlarged diameter annular section 1178 which together withcooperating hydrostatic pads forms a hydrostatic thrust bearing. Oil forthe hydrostatic pads of the journal bearing is provided via drillings1180 and 1182 while that for the pads of the thrust bearing viadrillings 1184 and 1186.

An air purge labyrinth seal is provided at 1188.

Seals are provided at 1190 and 1192 to ensure that there is nopossibility of oil leaking into the section containing the motorwindings.

Beyond the motor is a lockout 1194 to axialy secure the motor and atacho generator unit is driven by drive pins a shaft extension 11967secured by clamp screws 1196 to the end of the shaft 1175.

Beyond the tacho generator is mounted a balancing ring 1198 in whichgrub screws or the like can be fitted so as to balance the assembly andbeyond it is a resolver unit 1200 which commutates the brushless motor.The shaft extension is of considerable reduced diameter in the region ofthe resolver and extends to the left where it is contacted by earthingbrushes 1202 and 1204. FIG. 51 is a cross-section on the line XX of FIG.48 and shows the drillings which provide oil under pressure to the fourhydrostatic pads 1206, 1208, 1210 and 1212. A second drillingcommunicating with such pad as shown in FIG. 48 permits the oil pressurein each pad to be monitored.

Wheeldrive

FIGS. 52 to 54 illustrate the hydrostatic drive for the grinding wheel.

The wheel is shown at 28 and detail of the mounting of the wheel can beobtained from FIG. 53. The wheel is mounted on a hub 1214 which issecured to the main driving hub 1216 by means of bolts one of which isshown at 1218.

The wheel is secured to an outer flange of the hub 1214 by means ofbolts 1220 which are preferably formed from nylon or a similar plasticsmaterial.

Additionally the radial and axial surfaces of the hub 1214 at 1222 and1224 are lined with a ceramic film so as to electrically isolate thewheel 28 from the conductive material of the hub 1214 and the matingradial face of the driving hub 1216 is also lined with a ceramic film at1226 to electrically isolate the wheel 28 from that component also.

The driving hub is keyed to a tapered end to the main drive shaft 1228.A key is shown at 1230 and a central securing bolt is shown at 1232which retains the main driving hub in position.

Tapped drillings at 1234 and 1236 enable grub screws to be inserted forbalancing.

A labyrinth type seal is formed on the inside surface of the hub 1216 sothat the ingress of dirt and moisture to the main shaft 1228 is largelyprevented.

A first hydrostatic bearing is arranged in the region designated byreference numeral 1238 and a second hydrostatic bearing is arranged inthe region of the reference numeral 1240. Drillings for supplying oil tothe various pads around the shaft 1228 are provided in the casting andare shown in hidden detail in respect of the bearing 1238.

Beyond the second hyrostatic bearing 1240, seals are arranged at 1242and 1244 (see FIG. 52) to prevent oil seeping into the electric motorsection.

The electric motor comprises the rotor 1246 clamped to the shaft 1248and a stator and stator winding generally designated 1248 cooled by ahelical coolant fluid path 1250.

Beyond the motor the shaft is continued with reduced diameter throughand drives a resolver which commutates the brushless motor. The resolveris generally designated by reference numeral 1252. Earthing brushes at1254 and 1256 ensure that the shaft is earthed.

A balancing ring containing tapped drillings to receive grubscrews isprovided at 1258.

A sensor is located at the extreme left hand end of the shaft 1228 whichis generally designated 1260. The sensor is adapted to sense the firsttouch between the grinding wheel and a workpiece and to generate anelectrical signal indicating that the workpiece has been engaged. Asuitable sensor is one manufactured by Dittel and which involves the useof a piezoelectric stack.

In order to reduce electromagnetic interference with the resolver, amu-metal screen 1262 is provided between the motor and the resolver.

In FIG. 54 which is a cross-section on the line AA in FIG. 53, thedrillings for supplying oil to the six different hydrostatic pads aroundthe shaft 1228, are shown. Oil is supplied to the various drillings togalleries and manifolds in and surrounding the housing 1264.

X-axis Measurement

The position of the wheelhead table and therefore the wheel in relationto the rest of the machine (the X-axis) is determined by means of agrating and optical reading head details of which are found in FIGS. 55to 57. Mounted on the wheelhead table is a scale 1266 whilst attached tothe machine frame 20 is a reading head 1268. As the table movesbackwards and forwards so the scale 1266 moves relative to the readinghead 1268 and the latter produces electrical pulses corresponding to thegratings seen by the head. The signals may be decoded and used todetermine the precise position of the table relative to the frame 20.

As best seen in FIG. 57, a proximity switch 1270 is mounted on a framebest seen in FIG. 55, and identified by reference numeral 1272. Theswitch is tripped as a metal bracket 1274 moves past the switch as thetable approaches its rearmost position. The latter is determined whenthe microswitch 1276 is operated by a cam 1278. Forward movement of thetable eventually brings a second cam 1280 into contact with a secondmicroswitch 1282 denoting the maximum forward movement of the table.

Lifting and Indexing of Wheelhead

(i) Lifting

FIG. 58 shows details of the wheelhead lift and indexing mechanism item1120 of FIG. 45. The mechanism is intended to elevate the wheelheadassembly 1116 to a sufficient amount to disengage teeth of two geartooth rings so as to enable the actuator 1134 of FIG. 46 to rotate thewheelhead assembly 1116 about the axis 1118 in FIG. 45. After thedesired rotation has been achieved, the mechanism 1120 of FIG. 58 allowsthe wheelhead assembly 1116 to drop so that the teeth once again engageto hold and lock the wheelhead assembly 1116 in position. Referring toFIG. 58, the unit 1120 is located within a cylindrical aperture 1284situated centrally within the table 1090. The unit comprises a generallycylindrical housing having a composite vertical cylindrical bore 1286within which is located and is slidable a cylindrical member 1288. Thelower end of the member 1288 carries a piston 1290 which is sealinglyengaged on both its inner and outer diameters with the member 1288 at1292 and with the cylindrical wall 1294 by a seal 1296. The piston isdisplaceable vertically from the position shown by introducing oilthrough a connection 1298 and internal drillings 1300 into thecompartment 1302 below the piston. The elevation of the piston causesthe member 1288 to rise and to lift with it the platform 1116 carried atits upper end.

In order to provide for rotation of 1116 relative to the member 1288, aroller bearing assembly 1304 is situated between the upper end of thecylindrical member 1288 and the internal cylindrical aperture in theplatform at the lower end of the wheelhead assembly 1116.

The weight of the wheelhead assembly is taken by means of a thrustbearing 1306 located between the underside of the platform at the lowerend of the wheelhead assembly 1116 and an annular ring 1308 locatedaround the upper end of the cylindrical member 1288. The flat undersideof the annlar member 1308 is provided with an annular flat bearingsurface 1310 which is a clearance fit from a machined surface on a ringmember 1314 itself secured to the underside of the platform at the baseof the head assembly 1116 by means of threaded studs 1316.

The underside of the platform 1116 is formed with a ring of gear teethwhich engage complimentary teeth formed in a corresponding ring on thetop side of the metal casting of item 1090. The teeth are formed on twoannular ring members 1318 and 1320 respectively, the former beingattached by means of pins 1322 to the underside of the wheelheadassembly 1116 and the latter (ring 1320) being attached by means of pins1324 to the upper surface of casting 1090.

The pitch of the teeth is selected so as to be sufficiently fine toenable indexing of the unit 1116 relative to 1090 by sufficiently smallsteps.

In use oil is pumped through 1298 into the chamber 1302 to elevate thepiston 1290 and the cylindrical member 1288 so as to lift 1116 so thatthe teeth on ring 1318 are clear of the teeth on the ring 1320. In thiscondition the actuator 1134 (FIG. 46) can be used to rotate wheelheadassembly 1116 through the desired arc and thereafter oil is releasedfrom the chamber 1302 via pipe connection 1326 enabling the piston 1290to drop and thereby allowing the unit 1116 likewise to drop causing theteeth on ring 1318 to once again engage the teeth on 1320 therebypreventing continued rotation of the unit 1116.

(ii) Indexing mechanism

FIGS. 59 and 60 provide detail of the actuator for rotating thewheelhead assembly 1116.

The actuator selected is a rotary screw and nut device previouslydescribed with reference to FIG. 46 and is identified by referencenumeral 1134. An electric motor drive 1142 rotates the device so as toextend or retract rod 1138 which is attached to a driving block 1328mounted for rotation about a cylindrical pin 1330 which is rotatable andslidable up and down a cylindrical hole 1332 which is formed in themounting plate 1334 on which the wheelhead assembly is mounted.

Lifting of the assembly 1116 results in plate 1334 being raised whichallows the rod 1330 to drop under the action of a spring 1331 located atthe upper end of the rod 1330. This causes a tapered section of the rod1336 to engage a tapered wall section of the aperture through which therod 1330 passes in the block 1328. By providing the tapered shaft andaperture and providing for disengagement except when drive is to betransmitted to the table, effective decoupling between the drive and thetable is effected except when the drive is to move the table.

Diameter Measurement

FIG. 61 shows in side elevation one of two diameter controlling gaugeswhich are adapted to be mounted on the worktable in between theheadstock and tailstock. To this end each gauge includes a base 1340with clamping means generally designated 1342 by which the base can beclamped to the slideway of the worktable 638. Secured to the upper endof the base is a Movomatic gauge generally designated 1344 having upperand lower finger assemblies 1346 and 1348 respectively which are movableto engage a workpiece and determine the diameter thereof. The gauge is aproprietary item and services therefore as recommended by themanufacturer are provides by means of cables and pipes generallydesignated 1350.

Wheel Guard

FIGS. 62 to 65 provide details of the wheelguard assembly.

Essentially this comprises a narrow generally rectangular housinggenerally designated 1352 which is adapted to be fitted to the table onwhich the grinding wheel is mounted and which includes a door 1354hinged at 1356 and 1358 to the remainder of the housing to provide saidaccess to the wheel for mounting and demounting wheels.

The front of the housing is generally open but an adjustable cover 1360is hinged at 1362 and is adjustable relative to the remainder of thehousing by a nut and screw arrangement generally designated 1364 byrotation of a nut 1366. Rotation of the latter adjusts the angle of thecover 1360 and allows the cover to be set to a minimum distance from theedge of the wheel shown in dotted outline at 20.

The wheel 28 is shown in FIG. 63 from which it will be seen how thecover 1360 extends down over part of the circumference of the wheel.

FIGS. 64 and 65 show how coolant fluid can be applied to the edge of thewheel just below the lower end of the cover 1360.

To this end a bracket 1368 is attached to the left hand side of thecover 1360 as viewed in FIG. 63, to provide a mounting for a pipe 1370extending from a pump (not shown). The pipe 1370 is bent around at 1372and includes a pivotable union 1374 to allow a lower section of pipe1376 to be angularly adjustable from the position shown at 1376 in FIG.64 outwardly to the position shown at 1378 and inwardly to the positionshown at 1380 and all intermediate positions therebetween so that thelower end of the pipe 1376 can be positioned at precisely the requireddistance from the edge of a grinding wheel 28. The curved outline of thewheel 28 is typical of the size of a grinding wheel in the context ofthe machine and the adjoining circular outline 1382 is intended to showthe maximum diameter of a workpiece suitable for mounting on themachine.

FIG. 65 provides detail of the bracket 1370 and pivotable union 1374.

The lower end of the tube 1376 is provided with an adaptor plate 1384 towhich a jet or spray nozzle can be attached to provide the requisitespray pattern for coolant fluid pumped via the pipe 1370 to be sprayedonto the edge of the rotating grinding wheel 28 via the nozzle at thelower end of the tube 1376 just ahead of the point of engagement betweenthe grinding wheel and the workpiece.

Wheel Forming Unit

As described with reference to FIGS. 1 to 4, a wheel forming unit ismounted above the headstock housing 22 of FIG. 2 and is contained in thehousing 36. The unit is provided so as to dress and form a diamond wheelidentified by reference numeral 29 in FIG. 2 and by reference numeral952 in FIGS. 38 and 41 which is mounted on the headstock assembly 950(see FIG. 38). The wheel 29 is used to dress the grinding wheel 28 asrequired.

The external periphery of the diamond wheel 29 occasionally needs to beformed and to this end an EDM forming wheel 1386 is mounted above thediamond wheel 29 (shown in dotted outline in FIG. 66) and can be loweredinto contact with the diamond wheel 29 by means of a feed and retractmechanism generally designated 1388. The latter is mounted on the frontface of a triangular frame 1390 which is carried on a slideway generallydesignated 1392 for movement axis of rotation of the headstock andtherefore workpiece. Rotation of a handle 1394 at the rear of the frame1390, allows the frame to be moved along the slideway 1392 so as toenable the EDM wheel 1386 to be positioned over the diamond wheel 29, orretracted to the left, as shown in FIG. 6, back to the position shown indotted outline at 1396, so that the wheel is well clear of the diamondwheel 29. This enables the worktable to be moved to the right to bringthe diamond wheel 29 into registry with the grinding wheel 28.

Services for the advance and retract mechanism 1388 are conveyed via aflexible umbilical 1398 from a fixed termination 1400 to a termination1402 mounted on the frame 1390.

Microswitch 1404 cooperates with a ramp and cam 1406 to provide anelectrical interlock. This ensures that the worktable cannot be moved tothe right unless the microswitch 1404 has been operated by the cam 1406which only occurs when the frame 1390 and wheel 1386 have been withdrawnfully to the left hand side of FIG. 6 leaving the diamond wheel 29clear. FIG. 67 (which is an end elevation view of the assembly of FIG.66) shows the headstock mounting block 22 and diamond wheel 29 and EDMforming wheel 1386. The height controlling mechanism 1388 mayconveniently include a Mahr type 1300 probe and a servo drive wherebythe EDM wheel 1386 can be raised and lowered.

An earthing cable 1408 connects the wheel 1386 to the machine bed.

Dressing of Grinding Wheel

Electrolytic wheel dressing of the grinding wheel 28 is preferred andreference has already been made to the mounting of an appropriate unitat 37 on the cover 30 for the grinding wheel 28. Detail of theelectrolytic dressing device is shown in FIG. 68. Electrolyte issupplied to a manifold 1410 for supply via pipe 1412 from one side ofthe unit to the other. Drillings within the housing convey the liquidelectrolyte to a pair of drillings at 1414 and 1416. These exit into acurve channel between a pair of electrodes one of which is shown at 1418for locating on one side of the grinding wheel and the other behind 1418and hidden from view but of similar configuration for locating on theother side of the grinding wheel. The curved profile inside the twoelectrodes shown at 1420 is adjusted to the radius of the wheel.

In use the unit is adjusted so as to provide just the right clearancebetween the curved surface 1420 and the internal side cheeks of the twoelectrodes of which one is shown at 1418 and the wheel (not shown inFIG. 68) and to this end a knurled knob 1422 is provided for adjustingthe radial spacing and two knurled knobs 1424 and 1426 respectivelyprovide for lateral adjustment of the two electrodes.

The unit is secured to the wheelguard 1352 shown in FIG. 62 and thecutaway region 1428 shown in FIG. 62 serves to accommodate the rearmounting bracket 1430 shown in FIG. 68. The upper face of the wheelguard1352 is cut away to accommodate the electrodes and the unit is securedat its front end to an uncut away section of the wheelguard 1352 bymeans of fixing nut 1432.

Shoulder Measurement

FIG. 69 provides detail of the shoulder touch probe which can beprovided for measuring radial shoulders ground on the workpiece. Theprobe is adapted to be mounted on the wheelhead table and comprises anarcuate arm 1434 pivotable about a vertical axis 1436 by means of ahinge joint generally designated 1438 and about a horizontal axis 1440by means of a motor 1442. The latter is carried by a bracket 1444 whichis mounted on the wheelhead table 1446.

The arm 1434 can be swung from the operating position in which it isshown in FIG. 69 upwards through approximately 1200 to occupy anelevated parked position shown partly in dotted outline at 1448.Normally, the probe will occupy the position shown at 1448 duringgrinding but afer a shoulder has been ground and the shoulder is to bechecked, the grinding wheel is retracted, the probe arm 1434 is rotateddown into the position shown in FIG. 69 and the worktable shifted untilthe probe makes contact with the radial shoulder which has been ground.

The actual sensing part of the probe comprises a finger 1450 pivotallymounted about an axis 1452 at the end of an arm 1454. Electricalconnection to the probe 1450 is made via a cable 1456.

An end of travel stop 1458 is mounted on the front face of the wheelheadtable and an adjustable stop is provided at 1460 so that the lowerposition of the arm 1434 can be adjusted during the setting up of themachine so that the probe finger 1450 protrudes upwardly.

Except for the pivoting about the horizontal axis 1440, the pivotingabout the vertical axis 1436 and the pivoting of the finger 1450 aboutthe vertical axis 1452 is resisted using centering spring means, or thelike, so that resistance is needed to urge the pivotable component outof in-line alignment.

Active Worksteady

During grinding, the force between the wheel 28 and the workpiece 30 canresult in a deflection of the workpiece which can result in grindinginaccuracies. It is known to provide a worksteady or workrest whichabuts the workpiece generally opposite the point of engagement betweenthe wheel and the workpiece, and which is mounted in a manner which willresist any deflection of the workpiece.

FIG. 70 shows an improved worksteady generally designated 1462comprising a table 1464 mounted on the worktable and provided with aworkpiece engaging probe assembly 1468 which is moveable by a coarsedrive comprising an electric motor and ball screw 1470, and by a finedrive comprising one or more piezo cells such as 1472.

In use the probe 1468 is driven towards the workpiece by a motor drivenball screw 1470 until it is within 20-30 microns of the workpiecewhereafter continued advancement of the probe table 1462 is inhibited byoperation of a hydraulic clamp 1474. The probe assembly is engageablewith the workpiece by expansion of the piezo cell 1472 by theapplication of an appropriate voltage to the cell, which is justsufficient to move the probe means 1468 into contact with the workpiece30 so as to exert thereon a force equal and opposite to that exerted bythe grinding wheel 28.

The probe assembly conveniently comprises two shoes each of which isindependently movable by means of an associated piezo cell. The twoshoes are and arranged above and below a plane containing the grindingwheel and workpiece axes, the grinding plane, in two planes equallyinclined above and below the said grinding plane, and convergent on andintersecting the workpiece axis.

If appropriate equal voltages are applied to the two piezo cells, thetwo cells exert the same force on the workpiece, albeit from twoconvergent directions, and the resultant force is the sum of the twoforces exerted by the two shoes on the workpiece.

If the direction in which the worksteady force is to be applied iscoplanar with the grinding plane, equal voltages are applied to the twopiezo cells.

If the direction in which the worksteady force is to be applied is notcoplanar with the grinding plane, the appropriate tilting of thedirection of the force is achieved by altering the relative magnitudesof the voltages applied to the piezo cells.

The magnitude and direction of the force to be applied is determined bythe CPE controller 248 (see FIG. 7), from signals supplied from theheadstock and tailstock pressure transducers, such as 978 in the case ofthe headstock and 1082 in the case of the tailstock. Signals aresupplied via lines 1478 and 1480 to headstock force computing circuit1482 and tailstock force computing 1484 respectively. Signals may besupplied from each of six transducers around each bearing in each of theheadstock and tailstock. In this event, difference signals are computedwithin the units 1482 and 1484. Alternatively differential transducersmay be used to produce difference signals for each of the three pairs ofpads in each of the bearings in which event the units 1482 and 1484serve to process these difference signals into a resultant signal foreach of the headstock and tailstock.

The central controller 248 determines the magnitude and direction of theresultant of the two forces acting on the headstock and tailstock and inturn computes the forces needed to be applied via the upper and lowershoes of the probe assembly 1468, to counteract the forces exerted atthe headstock and tailstock by the engagement of the grinding wheel withthe workpiece.

Control signals for the motor 1462 and hydraulic clamp 1474 are derivedby motor drive circuit 1486 and solenoid valve assembly 1488respectively and electrical signals for extending the piezo cells suchas 1472 are derived by the control circuit 1490.

Typically the two shoes subtend an angle of at least 60°(ie 30°above and30° below the grinding plane) and conveniently the angle subtended is90° , ie 45° above and below the grinding plane.

Correction of X-axis Movement of the Workpiece Due to Z-axisImperfections

Correction of errors arising during Z-axis movement of the worktable andcaused for example by yaw and/or roll of the worktable can be correctedby moving the wheelhead along the Xaxis by an appropriate amount.

The worktable slides on a slideway and during setting up of the machineit is necessary to ensure that the worktable travel is orthogonal to thewheelfeed direction of movement by adjustment of the ceramic blocksforming the worktable slideways.

Use of Straight Edge on Worktable

As shown in FIGS. 23A/B in this machine a conductive straight edge (660,714) is mounted on the worktable to co-act with a conductive probe (710)forming with the straight edge a capacitance, the value of which will bedependent on the precise distance between the probe and the conductivesurface 714 of the straight edge.

As a first approximation the latter can be assumed to be perfectly flatand straight. By setting it up on the worktable so as to be parallel tothe worktable traverse (the Z-axis) using the adjustments provided suchas 770 (see FIG. 22), the capacitance value should not vary as the tableis moved from one end of its traverse to the other. (In fact at submicron levels this is impossible to achieve but the variation incapacitance due to non parallelism will be linear and can be identifiedand corrected for, see below). Fine adjustments can be made by adjusting770. Disregarding the linear variations, any other variation ofcapacitance noted with movement of the table along the Z-axis (assumingthe straight edge is flat) can be attributed to Z-axis/table mountingimperfections yaw and/or roll and will need to be corrected. This isachieved by generating an error signal equal to the variation ofcapacitance from the “normal” constant value, and adjusting for examplethe X-axis demand signal or the X-axis encoder signal to take account ofthe error signal.

Referring to the schematic circuit diagrams of FIGS. 71 and 72, controlof the wheelfeed is achieved by indicating the X-axis position requiredof the wheel and subtracting from this the X-axis position as determinedby the X-axis encoder (ie the optical reading head and scale mounted onthe wheelhead table see FIGS. 55 to 58) in device 1500. If there is anydifference between the two X values, an error signal is generated,enabling the X-axis wheelfeed drive 1168 (FIG. 48) until the errorsignal is reduced to zero, at which the wheelfeed stops. To this endFIGS. 71 and 72 show the output of amplifier 1504 supplying an input tothe X-axis drive measure 1168 which drives table 1090 and scale 1266past the reading head 1268 (see FIG. 56). A feedback amplfier 1269 isalso shown.

The introduction of the capacitance gauge error signal is achieved byintroducing a further adding device 1502 between the drive 1500 and theservo amplifier 1504. The basic position error signal from 1500 issupplied together with the error signal derived from the capacitancegauge to the device 1502. If the capacitance gauge signal is non-zero,then the X-axis drive 1168 (FIG. 48) will be enabled until the X-axisencoder reading (the reading head and scale on the wheelhead table seeFIGS. 55 to 57), produces a position error signal of sufficientmagnitude and sign so as to cancel out the capacitance gauge errorsignal, so terminating the wheelfeed drive once again.

By allowing this to happen in real time, so the wheelfeed will beenabled to incrementally adjust the wheelhead position along the X-axisto take account of any capacitance gauge error signals.

Typically the capacitance gauge is read at regularly spaced intervals oftime and the input to 1502 updated accordingly.

FIG. 72 is different from FIG. 71 in that it is the X-position encodersignal which is modified by the error signals before being combined in1500 with the X-axis demand signal. The net effect is the same as thearrangement shown in FIG. 71 and it is merely drawn in this way to showthe alternative way of handling the error signals.

If the straight edge flatness is not perfect, a calibration of theflatness relative to the length of the straight edge is stored in amemory 1506 as shown in FIG. 72, arranged as a look-up table based onZ-axis position for read-out addressing.

Using the Z-position information, the appropriate correction signal canbe read out from the memory for combination with the output from 1502for combining in a third adding device 1508.

Disregarding devices 1510 and 1516 for the moment it will be seen thatthe X-position encoder signal will be adjusted by means of 1508 and 1502before it is applied to 1500. This will enable the X-axis wheelfeeddrive until such time as the X-position encoder (the optical readinghead 1268 and scale 1266 on the wheelhead table 1090 (see FIGS. 55 to57)) produces an X-axis position value which is sufficient to cancel outthe error introduced by the signals from memory 1506 and the capacitancegauge.]

If X-axis errors are known to exist, a look-up table memory 1512 canstore these for different X and/or Z positions. Reading out andsupplying to adding device 1510 enables this further correction to bemade.

Workpiece Misalignment

The sliding engagement of the headstock and tailstock with the table issuch as to ensure that the workpiece axis should be parallel to theZ-axis slideway. Any error due to misalignment can be determined forexample after a single cylindrical grinding traverse using a diametermeasuring gauge such as shown in FIG. 61. If the workpiece axis is notparallel to the Z-axis slideway, the diameter of the workpiece willtaper towards one end. Since this is a straight line error it can becorrected using a simple algorithm of the form dx=kZ, where “k” willtend to be very small and dx is the X-axis displacement at any pointalong the Z-axis needed to compensate for the non-parallelism of theworkpiece. This algorithm may be used to plot error signals fordifferent Z-axis values for storage in memory 1516 for example.

Alternatively the device 1516 may be a processor set to process Z-axisvalues in real time to produce the corresponding values of dx using thealgorithm, for supply as error signals to adding device 1514.

The device 1514 thus serves to compensate for any non parallelism of theworkpiece and worktable travel.

Misalignment (Non-paralellism) of the Reference Straight Edge

Any “linear” variation of capacitance reading from the capacitance gaugefor different values of Z can be compensated for by adjusting thealgorithm to include this variation as well as the linear variation dueto workpiece misalignment (already dealt with). Alternatively a furtherlook-up memory 1520 may be used (or a processor with another algorithm)to generate error signals for supply to another adding device 1522 inthe line 1524 from device 1516 leading to the adding stage 1514.Alternatively 1522 could be located in the feedback path 1518 leadingfrom 1269 to 1514 so that like the other devices it is also in serieswith the feedback path.

As with the circuit change between FIGS. 71 and 72, the alternativearrangement is shown merely to indicate how error signals can becombined before they are used to effect the return signal or can eachindividually be used to effect the return signal. The net effect is thesame.

Calibration of Reference Straight Edge

Flatness/straightness of the reference straight edge may be measuredagainst a standard and set of calibration values relative to lengthobtained and stored.

Alternatively the capacitance gauge in the machine may be used tomeasure the capacitance variation as the table is traversed and thevalues plotted against the Z displacement. If the reference straightedge is then removed and rotated through 180° so that the conductivestrip is now facing the grinding wheel instead of the probe, and thestraight edge is then refitted to the worktable, it is possible to againmeasure the capacitance variation as the table is traversed by extendingthe capacitance probe using an appropriate bracket so as to reach overthe top of the reference straight edge so that the conductive electrodeis spaced from the conductive strip which is now facing the grindingwheel. Since it is important that the height at which the measurement ismade is constant, shins or spacers will be needed to lift the straightedge relative to the worktable when the straight edge has been rotatedas described so that the conductive strip is again at the same height asit is when the reference straight edge is normally mounted on theworktable a shown in the drawings.

Traversing the table in the same way as before allows a set ofcapacitance values to be obtained and these again are plotted on agraph.

The two plots will both start at 00 and will show a general drift awayfrom the X-axis of the graph (which corresponds to the Z-displacements)since the linear variation of capacitance due to non-paralellism of thestraight edge relative to the worktable line of traverse will tend togenerally increase or generally decrease the capacitance value. Anyvariation relative to the straight line drift will be brought abouteither as a result of inaccuracies in the worktable travel or due tovariation in the surface of the conductive strip.

The two plots need to be normalised and this is achieved by simplydrawing a straight line on each graph from the 00 point to the lastplotted value of capacitance. The values of capacitance above or belowthe line represent the actual variations of capacitance disregarding thedrift due to non-parallelism.

By adding the normalised plotted values for corresponding Z-axisdisplacement and dividing by 2, a true value of capacitance will beobtained for each Z-axis displacement.

Comparison of these values with the reference capacitance value (using abridge or the like device) allows the actual capacitance variation to bedetermined relating to the straight edge non-flatness. These values maythen be stored in the memory such as 1520.

What is claimed is:
 1. A machine tool, comprising: (a) a machine frame;(b) a worktable assembly slidably carried by said machine frame forreciprocal movement along a work axis defined by a rotational axis ofheadstock and tailstock means and which is configured to carry aworkpiece whereby the workpiece is movable along said work axis; (c) aworktool assembly carried by said machine frame for reciprocal movementalong a worktool axis at a predetermined angle with respect to said workaxis and so as to intersect same with a worktool movable along saidworktool axis; (d) a reference straight edge carried by said worktableassembly and having a reference surface; (e) reference sensing meanscarried by said machine frame for coaction with said reference straightedge reference surface, to measure reference spacing between saidreference surface and said reference sensing means and to provide areference output indicative of said reference spacing; and (f) controlmeans to facilitate movement of said worktool towards and away from saidwork axis which is responsive to said reference output to furtherfacilitate worktool movement towards said work axis, wherein (g) inorder to avoid parallax errors said reference spacing is measured at aheight above said worktable assembly which is commensurate with theheight of said work axis.
 2. The machine tool of claim 1, wherein apoint in said reference surface, at which said reference spacing ismeasured, the work axis and said worktool axis all lie in the sameplane.
 3. The machine tool of claim 2, wherein said worktool assemblyincludes a worktool carriage movably disposed on a slideway carried bysaid machine frame for movement along said worktool axis and worktoolcarriage position sensing means providing an output signal indicative ofthe position of said worktool carriage to said control means to furtherfacilitate movement of said worktool towards said work axis.
 4. Themachine too, of claim 3, wherein said worktool carriage position sensingmeans is located at same height as the said point in said referencesurface at which said reference spacing measurement occurs.
 5. Themachine tool of claim 4, wherein said worktool carriage position sensingmeans and said reference spacing measuring point are aligned with saidworktool axis.
 6. The machine tool of any of claims 2 to 5, wherein saidworktool is a grinding wheel rotatable about a wheel axis, and saidreference spacing measurement point, workpiece axis, worktool axis andsaid wheel axis all lie in the same plane.
 7. The machine tool of claim6, wherein the machine is mounted so that the said one plane ishorizontal.
 8. The machine tool of any of claims 1 and 2 to 5, whereinsaid control means includes a look-up table storing information relativeto initial dispositions of said reference surface and said referencesensing means and to provide an initial disposition output indicativethereof to modify said reference output to further facilitate worktoolmovement towards said work axis.
 9. The machine tool of claim 8, whereinsaid look-up table initial disposition output is indicative of theflatness of said reference surface.
 10. The machine tool of claim 9,wherein said reference sensing means includes a fixed electrode thatforms with said reference surface a first capacitance gauge.
 11. Themachine tool of claim 10, wherein a second capacitance gauge is disposedclose to said first capacitance gauge and a capacitance bridge isprovided including said first and second capacitance gauges, to providea capacitance variation signal.
 12. The machine tool of any of claims 1and 2 to 5, wherein said worktable assembly includes a work carriagemovably disposed on slideways carried by said machine frame for movementalong said work axis, and said worktool assembly carries a worktool, aworking face of which is positionable thereby with respect to said workaxis; said reference spacing being indicative of variations in spacingbetween said working face of said worktool and said work axis due toshifting of said work carriage with respect to said work face of saidwork tool.
 13. The machine tool of claim 3, wherein said worktoolcarriage position sensing means includes a grating disposed on saidworktool carriage in a worktool imaginary reference line that intersectswork when carried by said worktable assembly.
 14. The machine tool ofany of claims 1 to 5, and 13, wherein a rigid cover is mounted on theworktable to protect the reference sensing means, said cover beingspaced from the latter.
 15. The machine tool of any of claims 3 to 5,and 13, wherein a rigid cover is mounted on the worktool carriage,spaced from the position sensing means to protect the latter.
 16. Themachine tool of claim 12, including a cover assembly for said slidewaysmounted to said machine frame movable relative to and responsive tomovement of said work carriage but not connected to said work carriage.17. The machine tool of claim 16, wherein said cover assembly includes afirst cover portion mounted to said machine frame to one side of saidwork carriage and a second cover portion mounted to said machine frameto the other side of said work carriage.
 18. A machine tool as claimedin any of claims 2-5, and 13, in which the tool is a grinding wheel. 19.A machine tool comprising a bed, a worktable movable relative to saidbed along a linear path, a tool movable to engage a workpiece on saidworktable, tool drive means for advancing and retracting the tool alonga tool path and which is generally orthogonal to said linear path,worktable drive means for effecting movement of said worktable relativeto said bed along said linear path and, therefore, relative to saidtool, a reference straight edge carried by said worktable, referencesensing means fixed on said machine bed and which is stationary relativeto the straight edge and spaced therefrom by a nominal spacing generallyat the level of the path of movement of said worktable, and cooperatingtherewith to produce an electrical signal and circuit means associatedtherewith to generate from said electric signal an error signalindicative of any departure of the distance between said referencestraight edge and said reference sensing means from said nominal spacingtherebetween, memory means for storing a calibration of the flatness ofthe straight edge for points along its length, electrical signalgenerating means for generating a target position signal for said toolalong said tool path, tool position sensing means for generating anelectrical feedback signal indicative of the actual position of saidtool along said tool path, further electrical circuit means responsiveto said electrical signal from said reference sensing means and to acalibration signal from said memory means thereby to influence eithersaid tool path target position signal, or said feedback signal so as toenable said tool drive means to position said tool along a z-axis in amanner which compensates for any variation in the spacing between saidreference straight edge and said reference sensing means caused byworktable yaw and roll factors.
 20. A machine tool as claimed in claim19, in which said reference straight edge is formed from an elongateblock of ceramic material.
 21. A machine tool as claimed in claim 20, inwhich one face of said block is coated with a conductive material toform an electrode, and said reference sensing means includes aconductive plate spaced from said conductive face of said block andforming therewith a sensing capacitance the value of which will varywith movement of the worktable relative to said fixed conductive plateif the trajectory of said worktable is not a straight line.
 22. Amachine tool as claimed in claim 21, in which said conductive materialis hard chrome.
 23. A machine tool as claimed in any of claim 21,further comprising a reference capacitance of substantially the samenominal capacitance as that of said sensing capacitance formed betweensaid electrode and conductive surface of the reference straight edge,said reference capacitance being positioned close to said sensingcapacitance so that environmental variations such as changes oftemperature and humidity which can alter capacitance can be balanced outby comparing the two capacitances.
 24. A machine tool as claimed inclaim 23 in which said two capacitances form part of a bridge circuit sothat it is only signals indicative of changes of capacitance of saidsensing capacitance relative to said reference capacitance which aretransmitted as output signals indicative of variation of capacitance dueto non straight line motion of said worktable.
 25. A machine tool asclaimed in claim 23, wherein one of the electrodes of said referencecapacitance comprises a hard chrome metallising of a surface of a blockof ceramic material similar to that forming said reference straightedge.
 26. A machine tool as claimed in any of claims 19, and 20-25,wherein the machine tool is a grinding wheel and the workpiece ismounted between said headstock means and tailstock means mounted on saidworktable and in which at least said headstock is rotated by drive meansto rotate the workpiece.
 27. A machine tool as claimed in claim 20, inwhich a grinding wheel is mounted on a wheelhead slidable along awheelhead axis perpendicular to the axis of linear movement of saidworktable and positioned generally opposite the position of saidreference sensing means which cooperates with said reference straightedge on said worktable, and wherein a wheelhead servo control system issupplied with a target signal defining the desired position of saidgrinding wheel along said wheelhead axis, signals obtained from saidreference sensing means associated with said reference straight edge,for combining with a feedback signal indicating the actual position ofsaid wheelhead, whereby said wheelhead is positionable along saidwheelhead axis relative to said worktable and therefore a workpiece,taking into account any alteration in spacing between said referencesensing means and said reference straight edge caused by z-axis tableyaw and roll imperfections.
 28. A machine tool as claimed in claim 27,further comprising look-up table means containing calibration datarelative to length of said reference straight edge, relating to linearvariation of the said spacing caused by non-parallel mounting of saidreference straight edge on said worktable and/or worktable and slideway,and/or linear or non-linear X-axis imperfections dependent on saidposition of said wheelhead on the X-axis.
 29. The combination with amachine tool and an apparatus for detecting unwanted lateral movement ofa worktable for carrying a workpiece and movable relative to a machinebed along a worktable axis defined by the rotational axis of a headstockand tailstock means, comprising a straight edge carried by saidworktable and a conductive probe fixed to said machine bed andcooperating with said straight edge generally at the level of the pathof movement of the workpiece, to form a capacitance which varies withany lateral movement of said worktable but maintains a predictable valuewith movement of said worktable along said worktable axis, and electriccircuit means coupled to the said capacitance to generate an electricsignal indicative of unpredicted variations in the said capacitance toprovide an electric signal indicative of any lateral movement of saidworktable relative to said worktable axis.
 30. The combination asclaimed in claim 29, further comprising means responsive to the saidelectrical signal derived from said capacitance variation to adjust theposition of a tool adapted to engage a workpiece carried on saidworktable so as to compensate for unwanted lateral movement of saidworktable.
 31. The combination as claimed in claim 30, furthercomprising a reference capacitance situated in close proximity to saidcapacitance formed between said straight edge and the said conductiveprobe to enable variations in capacitance of the latter due toenvironmental changes to be balanced out.
 32. A method of controllingthe position of a machining tool relative to a workpiece mounted on aworktable itself slidable in a straight line along a worktable axisdefined by a rotational axis of headstock and tailstock means relativeto a machine bed, in a direction generally perpendicular to thedirection of movement of said machining tool, said worktable carrying areference straight edge which cooperates with sensing means fixed tosaid machine bed and located generally opposite the point of applicationof said machining tool generally at the level of said workpiece axis,for determining any non-straight line movement of said worktable,comprising the steps of: storing a calibration signal in the form of alook up table for a plurality of positions of said worktable along saidworktable axis; generating an error signal indicative of anynon-straight line movement of said worktable; adjusting said errorsignal using the stored calibration signals; and adjusting a signaldefining the target position for said machining tool by said adjustederror signal thereby to cause tool positioning means to position thesaid machining tool relative to said worktable and any workpiecepositioned thereon taking into account errors effecting the accuracy ofthe tool position relative to said worktable axis due to yaw, roll ornon-parralism of said straight edge to said z-axis.
 33. A method asclaimed in claim 32, wherein said machining tool is a grinding wheelmounted on a wheelhead which is slidable in a direction generallyperpendicular to the direction of movement of said worktable.
 34. Amethod as claimed in claim 32, in which said reference straight edge iselectrically conductive and forms with a fixed electrode a capacitancewhich varies if said worktable does not follow a straight line path andwherein circuit means is provided for generating an electrical signalindicative of any capacitance variation thereby to generate an errorsignal for influencing the value of a wheelhead target position signalor a wheelhead position feedback signal to enable the wheelhead positionmeans, to adjust the position thereof, relative to said worktable, totake account of said lateral movement of the worktable sensed by thereference straight edge sensing means.
 35. A method of correcting errorsdue to unwanted lateral yaw and roll movement of a worktable motion in amachine tool in which the worktable for carrying a workpiece is movablealong a straight line path relative to a machine bed along a work axiswhich is defined by the rotational axis of headstock and tailstockmeans, the worktable carries a reference straight edge substantiallyparallel to the work axis and the machine bed carries a sensor adaptedto cooperate with the reference straight edge generally at the level ofthe path of movement of the workpiece and forms a transducer therewithwhich will vary an electric signal in the event of lateral movement ofthe worktable along its path, the method comprising the steps of:monitoring the said electric signal and generating an error signaldependent on any variation therein, the said error signal indicating inmagnitude and sign lateral movement of the worktable from a straightline path intended for it to follow, and using said error signal tocontrol the final position of the tool movable relative to the worktableand therefore any workpiece located thereon.
 36. A method as claimed inclaim 35, wherein the transducer is a capacitance formed betweenconductive surfaces on the reference straight edge and a final electrodeand further comprising the step of providing a second capacitance theelectrodes of which are located in close proximity to the firstcapacitance, to provide a reference capacitance for comparison with thefirst said capacitance and monitoring both said capacitances to enableenvironmentally induced changes in the said first capacitance to bebalanced out by the same induced changes in the reference capacitance.37. The machine tool of claim 8, wherein said worktool is a grindingwheel rotatable about a wheel axis, and said reference spacingmeasurement point, workpiece axis, worktool axis and said wheel axis alllie in the same plane.
 38. The machine tool of claim 37, wherein themachine is mounted so that the said one plane is horizontal.
 39. Themachine tool of claim 9, wherein said worktool is a grinding wheelrotatable about a wheel axis, and said reference spacing measurementpoint, workpiece axis, worktool axis and said wheel axis all lie in thesame plane.
 40. The machine tool of claim 10, wherein said worktool is agrinding wheel rotatable about a wheel axis, and said reference spacingmeasurement point, workpiece axis, worktool axis and said wheel axis alllie in the same plane.
 41. The machine tool of claim 11, wherein saidworktool is a grinding wheel rotatable about a wheel axis, and saidreference spacing measurement point, workpiece axis, worktool axis andsaid wheel axis all lie in the same plane.
 42. The machine tool of claim12, wherein said worktool is a grinding wheel rotatable about a wheelaxis, and said reference spacing measurement point, workpiece axis,worktool axis and said wheel axis all lie in the same plane.
 43. Themachine tool of claim 42 wherein the machine is mounted so that the saidone plane is horizontal.
 44. The machine tool of claim 12, wherein saidcontrol means includes a look-up table storing information relative toinitial dispositions of said reference surface and said referencesensing means and to provide an initial disposition output indicativethereof to modify said reference output to further facilitate worktoolmovement towards said work axis.
 45. The machine tool of claim 44,wherein said worktool is a grinding wheel rotatable about a wheel axis,and said reference spacing measurement point, workpiece axis, worktoolaxis and said wheel axis lie in the same plane.
 46. The machine tool ofclaim 44, wherein said look-up table initial disposition output isindicative of the flatness of said reference surface.
 47. The machinetool of claim 46, wherein said worktool is a grinding wheel rotatableabout a wheel axis, and said reference spacing measurement point,workpiece axis, worktool axis and said wheel axis lie in the same plane.48. The machine tool of claim 46, wherein said reference sensing meansincludes a fixed electrode that forms with said reference surface afirst capacitance gauge.
 49. The machine tool of claim 48, wherein saidworktool is a grinding wheel rotatable about a wheel axis, and saidreference spacing measurement point, workpiece axis, worktool axis andsaid wheel axis lie in the same plane.
 50. The machine tool of claim 48,wherein a second capacitance gauge is disposed close to said firstcapacitance gauge and a capacitance bridge is provided including saidfirst and second capacitance gauges, to provide a capacitance variationsignal.
 51. The machine tool of claim 50, wherein said worktool is agrinding wheel rotatable about a wheel axis, and said reference spacingmeasurement point, workpiece axis, worktool axis and said wheel axis liein the same plane.
 52. The machine tool of claim 51, wherein the machineis mounted so that the said one plane is horizontal.
 53. The machinetool of claim 14, wherein said worktool is a grinding wheel rotatableabout a wheel axis, and said reference spacing measurement point,workpiece axis, worktool axis and said wheel axis lie in the same plane.54. The machine tool of claim 53, wherein the machine is mounted so thatthe said one plane is horizontal.
 55. The machine tool of claim 14,wherein said control means includes a look-up table storing informationrelative to initial dispositions of said reference surface and saidreference sensing means and to provide an initial disposition outputindicative thereof to modify said reference output to further facilitateworktool movement towards said work axis.
 56. The machine tool of claim55, wherein said look-up table initial disposition output is indicativeof the flatness of said reference surface.
 57. The machine tool of claim56, wherein said reference sensing means includes a fixed electrode thatforms with said reference surface a first capacitance gauge.
 58. Themachine tool of claim 57, wherein a second capacitance gauge is disposedclose to said first capacitance gauge and a capacitance bridge isprovided including said first and second capacitance gauges, to providea capacitance variation signal.
 59. The machine tool of claim 14,wherein said worktable assembly includes a work carriage movablydisposed on slideways carried by said machine frame for movement alongsaid work axis, and said worktool assembly carries a worktool, a workingface of which is positionable thereby with respect to said work axis;said reference spacing being indicative of variations in spacing betweensaid working face of said worktool and said work axis due to shifting ofsaid work carriage with respect to said work face of said work tool. 60.The machine tool of claim 15, wherein said worktool is a grinding wheelrotatable about a wheel axis, and said reference spacing measurementpoint, workpiece axis, worktool axis and said wheel axis lie in the sameplane.
 61. The machine tool of claim 60, wherein the machine is mountedso that the said one plane is horizontal.
 62. The machine tool of claim15, wherein said control means includes a look-up table storinginformation relative to initial dispositions of said reference surfaceand said reference sensing means and to provide an initial dispositionoutput indicative thereof to modify said reference output to furtherfacilitate worktool movement towards said work axis.
 63. The machinetool of claim 62, wherein said look-up table initial disposition outputis indicative of the flatness of said reference surface.
 64. The machinetool of claim 63, wherein said reference sensing means includes a fixedelectrode that forms with said reference surface a first capacitancegauge.
 65. The machine tool of claim 64, wherein a second capacitancegauge is disposed close to said first capacitance gauge and acapacitance bridge is provided including said first and secondcapacitance gauges, to provide a capacitance variation signal.
 66. Themachine tool of claim 15, wherein said worktable assembly includes awork carriage movably disposed on slideways carried by said machineframe for movement along said work axis, and said worktool assemblycarries a worktool, a working face of which is positionable thereby withrespect to said work axis; said reference spacing being indicative ofvariations in spacing between said working face of said worktool andsaid work axis due to shifting of said work carriage with respect tosaid work face of said work tool.